diff --git a/simplexmq.cabal b/simplexmq.cabal
index b7160fb05..b4923024c 100644
--- a/simplexmq.cabal
+++ b/simplexmq.cabal
@@ -103,6 +103,7 @@ library
       Simplex.Messaging.Agent.Store.SQLite.Migrations.M20231225_failed_work_items
       Simplex.Messaging.Agent.TAsyncs
       Simplex.Messaging.Agent.TRcvQueues
+      Simplex.Messaging.Builder
       Simplex.Messaging.Client
       Simplex.Messaging.Client.Agent
       Simplex.Messaging.Crypto
diff --git a/src/Simplex/FileTransfer/Client.hs b/src/Simplex/FileTransfer/Client.hs
index 9489f52c1..69896408c 100644
--- a/src/Simplex/FileTransfer/Client.hs
+++ b/src/Simplex/FileTransfer/Client.hs
@@ -13,7 +13,7 @@ import Control.Monad
 import Control.Monad.Except
 import Crypto.Random (ChaChaDRG)
 import Data.Bifunctor (first)
-import Data.ByteString.Builder (Builder, byteString)
+import qualified Data.ByteString.Builder as BB
 import Data.ByteString.Char8 (ByteString)
 import qualified Data.ByteString.Char8 as B
 import Data.Int (Int64)
@@ -25,6 +25,7 @@ import qualified Network.HTTP2.Client as H
 import Simplex.FileTransfer.Description (mb)
 import Simplex.FileTransfer.Protocol
 import Simplex.FileTransfer.Transport
+import Simplex.Messaging.Builder (Builder, builder)
 import Simplex.Messaging.Client
   ( NetworkConfig (..),
     ProtocolClientError (..),
@@ -138,7 +139,7 @@ sendXFTPCommand c@XFTPClient {http2Client = HTTP2Client {sessionId}} pKey fId cm
       xftpEncodeTransmission sessionId (Just pKey) ("", fId, FileCmd (sFileParty @p) cmd)
   sendXFTPTransmission c t chunkSpec_
 
-sendXFTPTransmission :: XFTPClient -> ByteString -> Maybe XFTPChunkSpec -> ExceptT XFTPClientError IO (FileResponse, HTTP2Body)
+sendXFTPTransmission :: XFTPClient -> Builder -> Maybe XFTPChunkSpec -> ExceptT XFTPClientError IO (FileResponse, HTTP2Body)
 sendXFTPTransmission XFTPClient {config, http2Client = http2@HTTP2Client {sessionId}} t chunkSpec_ = do
   let req = H.requestStreaming N.methodPost "/" [] streamBody
       reqTimeout = (\XFTPChunkSpec {chunkSize} -> chunkTimeout config chunkSize) <$> chunkSpec_
@@ -152,9 +153,9 @@ sendXFTPTransmission XFTPClient {config, http2Client = http2@HTTP2Client {sessio
       _ -> pure (r, body)
     Left e -> throwError $ PCEResponseError e
   where
-    streamBody :: (Builder -> IO ()) -> IO () -> IO ()
+    streamBody :: (BB.Builder -> IO ()) -> IO () -> IO ()
     streamBody send done = do
-      send $ byteString t
+      send $ builder t
       forM_ chunkSpec_ $ \XFTPChunkSpec {filePath, chunkOffset, chunkSize} ->
         withFile filePath ReadMode $ \h -> do
           hSeek h AbsoluteSeek $ fromIntegral chunkOffset
diff --git a/src/Simplex/FileTransfer/Protocol.hs b/src/Simplex/FileTransfer/Protocol.hs
index 58392d685..19d458107 100644
--- a/src/Simplex/FileTransfer/Protocol.hs
+++ b/src/Simplex/FileTransfer/Protocol.hs
@@ -24,6 +24,7 @@ import Data.List.NonEmpty (NonEmpty (..))
 import Data.Maybe (isNothing)
 import Data.Type.Equality
 import Data.Word (Word32)
+import Simplex.Messaging.Builder (Builder)
 import qualified Simplex.Messaging.Crypto as C
 import Simplex.Messaging.Encoding
 import Simplex.Messaging.Encoding.String
@@ -394,7 +395,7 @@ checkParty' c = case testEquality (sFileParty @p) (sFileParty @p') of
   Just Refl -> Just c
   _ -> Nothing
 
-xftpEncodeTransmission :: ProtocolEncoding e c => SessionId -> Maybe C.APrivateSignKey -> Transmission c -> Either TransportError ByteString
+xftpEncodeTransmission :: ProtocolEncoding e c => SessionId -> Maybe C.APrivateSignKey -> Transmission c -> Either TransportError Builder
 xftpEncodeTransmission sessionId pKey (corrId, fId, msg) = do
   let t = encodeTransmission currentXFTPVersion sessionId (corrId, fId, msg)
   xftpEncodeBatch1 $ signTransmission t
@@ -403,10 +404,10 @@ xftpEncodeTransmission sessionId pKey (corrId, fId, msg) = do
     signTransmission t = ((`C.sign` t) <$> pKey, t)
 
 -- this function uses batch syntax but puts only one transmission in the batch
-xftpEncodeBatch1 :: (Maybe C.ASignature, ByteString) -> Either TransportError ByteString
+xftpEncodeBatch1 :: (Maybe C.ASignature, ByteString) -> Either TransportError Builder
 xftpEncodeBatch1 (sig, t) =
-  let t' = tEncodeBatch 1 . smpEncode . Large $ tEncode (sig, t)
-   in first (const TELargeMsg) $ C.pad t' xftpBlockSize
+  let t' = tEncodeBatch 1 . encodeLarge $ tEncode (sig, t)
+   in first (const TELargeMsg) $ C.pad' t' xftpBlockSize
 
 xftpDecodeTransmission :: ProtocolEncoding e c => SessionId -> ByteString -> Either XFTPErrorType (SignedTransmission e c)
 xftpDecodeTransmission sessionId t = do
diff --git a/src/Simplex/FileTransfer/Server.hs b/src/Simplex/FileTransfer/Server.hs
index f4b725462..c1fc4be41 100644
--- a/src/Simplex/FileTransfer/Server.hs
+++ b/src/Simplex/FileTransfer/Server.hs
@@ -18,7 +18,6 @@ import Control.Monad.Except
 import Control.Monad.Reader
 import Data.Bifunctor (first)
 import qualified Data.ByteString.Base64.URL as B64
-import Data.ByteString.Builder (byteString)
 import Data.ByteString.Char8 (ByteString)
 import qualified Data.ByteString.Char8 as B
 import Data.Functor (($>))
@@ -44,6 +43,7 @@ import Simplex.FileTransfer.Server.Stats
 import Simplex.FileTransfer.Server.Store
 import Simplex.FileTransfer.Server.StoreLog
 import Simplex.FileTransfer.Transport
+import Simplex.Messaging.Builder (builder)
 import qualified Simplex.Messaging.Crypto as C
 import qualified Simplex.Messaging.Crypto.Lazy as LC
 import Simplex.Messaging.Encoding.String
@@ -242,7 +242,7 @@ processRequest HTTP2Request {sessionId, reqBody = body@HTTP2Body {bodyHead}, sen
               send "padding error" -- TODO respond with BLOCK error?
               done
             Right t -> do
-              send $ byteString t
+              send $ builder t
               -- timeout sending file in the same way as receiving
               forM_ serverFile_ $ \ServerFile {filePath, fileSize, sbState} -> do
                 withFile filePath ReadMode $ \h -> sendEncFile h send sbState (fromIntegral fileSize)
diff --git a/src/Simplex/Messaging/Builder.hs b/src/Simplex/Messaging/Builder.hs
new file mode 100644
index 000000000..31e94bf7d
--- /dev/null
+++ b/src/Simplex/Messaging/Builder.hs
@@ -0,0 +1,52 @@
+{-# LANGUAGE StrictData #-}
+
+module Simplex.Messaging.Builder
+  ( Builder (length, builder),
+    byteString,
+    lazyByteString,
+    word16BE,
+    char8,
+    toLazyByteString,
+  )
+where
+
+import qualified Data.ByteString as B
+import qualified Data.ByteString.Builder as BB
+import qualified Data.ByteString.Lazy as LB
+import Data.Word (Word16)
+
+
+-- length-aware builder
+data Builder = Builder {length :: Int, builder :: BB.Builder}
+
+instance Semigroup Builder where
+  Builder l1 b1 <> Builder l2 b2 = Builder (l1 + l2) (b1 <> b2)
+  {-# INLINE (<>) #-}
+
+instance Monoid Builder where
+  mempty = Builder 0 mempty
+  {-# INLINE mempty #-}
+  mconcat bs = Builder (sum ls) (mconcat bbs)
+    where
+      (ls, bbs) = foldr (\(Builder l b) ~(ls', bbs') -> (l : ls', b : bbs')) ([], []) bs
+  {-# INLINE mconcat #-}
+
+byteString :: B.ByteString -> Builder
+byteString s = Builder (B.length s) (BB.byteString s)
+{-# INLINE byteString #-}
+
+lazyByteString :: LB.ByteString -> Builder
+lazyByteString s = Builder (fromIntegral $ LB.length s) (BB.lazyByteString s)
+{-# INLINE lazyByteString #-}
+
+word16BE :: Word16 -> Builder
+word16BE = Builder 2 . BB.word16BE
+{-# INLINE word16BE #-}
+
+char8 :: Char -> Builder
+char8 = Builder 1 . BB.char8
+{-# INLINE char8 #-}
+
+toLazyByteString :: Builder -> LB.ByteString
+toLazyByteString = BB.toLazyByteString . builder
+{-# INLINE toLazyByteString #-}
diff --git a/src/Simplex/Messaging/Client.hs b/src/Simplex/Messaging/Client.hs
index bddce8f34..1d941ca08 100644
--- a/src/Simplex/Messaging/Client.hs
+++ b/src/Simplex/Messaging/Client.hs
@@ -72,9 +72,7 @@ module Simplex.Messaging.Client
     ClientCommand,
 
     -- * For testing
-    ClientBatch (..),
     PCTransmission,
-    batchClientTransmissions,
     mkTransmission,
     clientStub,
   )
@@ -98,11 +96,13 @@ import Data.Maybe (fromMaybe)
 import Data.Time.Clock (UTCTime, getCurrentTime)
 import Network.Socket (ServiceName)
 import Numeric.Natural
+import Simplex.Messaging.Builder (Builder)
+import qualified Simplex.Messaging.Builder as BB
 import qualified Simplex.Messaging.Crypto as C
 import Simplex.Messaging.Encoding
 import Simplex.Messaging.Encoding.String
 import Simplex.Messaging.Parsers (defaultJSON, dropPrefix, enumJSON)
-import Simplex.Messaging.Protocol as SMP
+import Simplex.Messaging.Protocol
 import Simplex.Messaging.TMap (TMap)
 import qualified Simplex.Messaging.TMap as TM
 import Simplex.Messaging.Transport
@@ -136,7 +136,7 @@ data PClient err msg = PClient
     pingErrorCount :: TVar Int,
     clientCorrId :: TVar Natural,
     sentCommands :: TMap CorrId (Request err msg),
-    sndQ :: TBQueue ByteString,
+    sndQ :: TBQueue Builder,
     rcvQ :: TBQueue (NonEmpty (SignedTransmission err msg)),
     msgQ :: Maybe (TBQueue (ServerTransmission msg))
   }
@@ -173,7 +173,7 @@ clientStub sessionId = do
             }
       }
 
-type SMPClient = ProtocolClient ErrorType SMP.BrokerMsg
+type SMPClient = ProtocolClient ErrorType BrokerMsg
 
 -- | Type for client command data
 type ClientCommand msg = (Maybe C.APrivateSignKey, EntityId, ProtoCommand msg)
@@ -634,7 +634,7 @@ type PCTransmission err msg = (SentRawTransmission, Request err msg)
 -- | Send multiple commands with batching and collect responses
 sendProtocolCommands :: forall err msg. ProtocolEncoding err (ProtoCommand msg) => ProtocolClient err msg -> NonEmpty (ClientCommand msg) -> IO (NonEmpty (Response err msg))
 sendProtocolCommands c@ProtocolClient {batch, blockSize} cs = do
-  bs <- batchClientTransmissions batch blockSize <$> mapM (mkTransmission c) cs
+  bs <- batchTransmissions' batch blockSize <$> mapM (mkTransmission c) cs
   validate . concat =<< mapM (sendBatch c) bs
   where
     validate :: [Response err msg] -> IO (NonEmpty (Response err msg))
@@ -651,58 +651,22 @@ sendProtocolCommands c@ProtocolClient {batch, blockSize} cs = do
 
 streamProtocolCommands :: forall err msg. ProtocolEncoding err (ProtoCommand msg) => ProtocolClient err msg -> NonEmpty (ClientCommand msg) -> ([Response err msg] -> IO ()) -> IO ()
 streamProtocolCommands c@ProtocolClient {batch, blockSize} cs cb = do
-  bs <- batchClientTransmissions batch blockSize <$> mapM (mkTransmission c) cs
+  bs <- batchTransmissions' batch blockSize <$> mapM (mkTransmission c) cs
   mapM_ (cb <=< sendBatch c) bs
 
-sendBatch :: ProtocolClient err msg -> ClientBatch err msg -> IO [Response err msg]
+sendBatch :: ProtocolClient err msg -> TransportBatch (Request err msg) -> IO [Response err msg]
 sendBatch c@ProtocolClient {client_ = PClient {sndQ}} b = do
   case b of
-    CBLargeTransmission Request {entityId} -> do
+    TBLargeTransmission Request {entityId} -> do
       putStrLn "send error: large message"
       pure [Response entityId $ Left $ PCETransportError TELargeMsg]
-    CBTransmissions s n rs -> do
+    TBTransmissions s n rs -> do
       when (n > 0) $ atomically $ writeTBQueue sndQ $ tEncodeBatch n s
       mapConcurrently (getResponse c) rs
-    CBTransmission s r -> do
+    TBTransmission s r -> do
       atomically $ writeTBQueue sndQ s
       (: []) <$> getResponse c r
 
-data ClientBatch err msg
-  = -- ByteString in CBTransmissions does not include count byte, it is added by tEncodeBatch
-    CBTransmissions ByteString Int [Request err msg]
-  | CBTransmission ByteString (Request err msg)
-  | CBLargeTransmission (Request err msg)
-
--- | encodes and batches transmissions into blocks
-batchClientTransmissions :: forall err msg. Bool -> Int -> NonEmpty (PCTransmission err msg) -> [ClientBatch err msg]
-batchClientTransmissions batch blkSize
-  | batch = reverse . mkBatch []
-  | otherwise = map mkBatch1 . L.toList
-  where
-    mkBatch :: [ClientBatch err msg] -> NonEmpty (PCTransmission err msg) -> [ClientBatch err msg]
-    mkBatch bs ts =
-      let (b, ts_) = encodeBatch "" 0 [] ts
-          bs' = b : bs
-       in maybe bs' (mkBatch bs') ts_
-    mkBatch1 :: PCTransmission err msg -> ClientBatch err msg
-    mkBatch1 (t, r)
-      | B.length s <= blkSize - 2 = CBTransmission s r
-      | otherwise = CBLargeTransmission r
-      where
-        s = tEncode t
-    encodeBatch :: ByteString -> Int -> [Request err msg] -> NonEmpty (PCTransmission err msg) -> (ClientBatch err msg, Maybe (NonEmpty (PCTransmission err msg)))
-    encodeBatch s n rs ts@((t, r) :| ts_)
-      | B.length s' <= blkSize - 3 && n < 255 =
-          case L.nonEmpty ts_ of
-            Just ts' -> encodeBatch s' n' rs' ts'
-            Nothing -> (CBTransmissions s' n' (reverse rs'), Nothing)
-      | n == 0 = (CBLargeTransmission r, L.nonEmpty ts_)
-      | otherwise = (CBTransmissions s n (reverse rs), Just ts)
-      where
-        s' = s <> smpEncode (Large $ tEncode t)
-        n' = n + 1
-        rs' = r : rs
-
 -- | Send Protocol command
 sendProtocolCommand :: forall err msg. ProtocolEncoding err (ProtoCommand msg) => ProtocolClient err msg -> Maybe C.APrivateSignKey -> EntityId -> ProtoCommand msg -> ExceptT (ProtocolClientError err) IO msg
 sendProtocolCommand c@ProtocolClient {client_ = PClient {sndQ}, batch, blockSize} pKey entId cmd =
@@ -711,11 +675,11 @@ sendProtocolCommand c@ProtocolClient {client_ = PClient {sndQ}, batch, blockSize
     -- two separate "atomically" needed to avoid blocking
     sendRecv :: SentRawTransmission -> Request err msg -> IO (Either (ProtocolClientError err) msg)
     sendRecv t r
-      | B.length s > blockSize - 2 = pure $ Left $ PCETransportError TELargeMsg
+      | BB.length s > blockSize - 2 = pure $ Left $ PCETransportError TELargeMsg
       | otherwise = atomically (writeTBQueue sndQ s) >> response <$> getResponse c r
       where
         s
-          | batch = tEncodeBatch 1 . smpEncode . Large $ tEncode t
+          | batch = tEncodeBatch 1 . encodeLarge $ tEncode t
           | otherwise = tEncode t
 
 -- TODO switch to timeout or TimeManager that supports Int64
diff --git a/src/Simplex/Messaging/Crypto.hs b/src/Simplex/Messaging/Crypto.hs
index 3564454ff..d6a24f772 100644
--- a/src/Simplex/Messaging/Crypto.hs
+++ b/src/Simplex/Messaging/Crypto.hs
@@ -140,6 +140,7 @@ module Simplex.Messaging.Crypto
 
     -- * Message padding / un-padding
     pad,
+    pad',
     unPad,
 
     -- * X509 Certificates
@@ -205,6 +206,8 @@ import Database.SQLite.Simple.FromField (FromField (..))
 import Database.SQLite.Simple.ToField (ToField (..))
 import GHC.TypeLits (ErrorMessage (..), KnownNat, Nat, TypeError, natVal, type (+))
 import Network.Transport.Internal (decodeWord16, encodeWord16)
+import Simplex.Messaging.Builder (Builder, byteString, word16BE)
+import qualified Simplex.Messaging.Builder as BB
 import Simplex.Messaging.Encoding
 import Simplex.Messaging.Encoding.String
 import Simplex.Messaging.Parsers (blobFieldDecoder, parseAll, parseString)
@@ -919,6 +922,14 @@ pad msg paddedLen
     len = B.length msg
     padLen = paddedLen - len - 2
 
+pad' :: Builder -> Int -> Either CryptoError Builder
+pad' msg paddedLen
+  | len <= maxMsgLen && padLen >= 0 = Right $ word16BE (fromIntegral len) <> msg <> byteString (B.replicate padLen '#')
+  | otherwise = Left CryptoLargeMsgError
+  where
+    len = BB.length msg
+    padLen = paddedLen - len - 2
+
 unPad :: ByteString -> Either CryptoError ByteString
 unPad padded
   | B.length lenWrd == 2 && B.length rest >= len = Right $ B.take len rest
diff --git a/src/Simplex/Messaging/Crypto/Lazy.hs b/src/Simplex/Messaging/Crypto/Lazy.hs
index ebb3692c1..c83d93a07 100644
--- a/src/Simplex/Messaging/Crypto/Lazy.hs
+++ b/src/Simplex/Messaging/Crypto/Lazy.hs
@@ -41,7 +41,6 @@ import qualified Crypto.MAC.Poly1305 as Poly1305
 import Data.Bifunctor (first)
 import Data.ByteArray (ByteArrayAccess)
 import qualified Data.ByteArray as BA
-import qualified Data.ByteString as S
 import Data.ByteString.Char8 (ByteString)
 import qualified Data.ByteString.Char8 as B
 import qualified Data.ByteString.Lazy.Char8 as LB
@@ -175,7 +174,7 @@ secretBoxTailTag sbProcess secret nonce msg = run <$> sbInit_ secret nonce
 
 -- passes lazy bytestring via initialized secret box returning the reversed list of chunks
 secretBoxLazy_ :: (SbState -> ByteString -> (ByteString, SbState)) -> SbState -> LazyByteString -> ([ByteString], SbState)
-secretBoxLazy_ sbProcess state = foldlChunks update ([], state)
+secretBoxLazy_ sbProcess state = LB.foldlChunks update ([], state)
   where
     update (cs, st) chunk = let (!c, !st') = sbProcess st chunk in (c : cs, st')
 
@@ -231,10 +230,3 @@ cryptoPassed :: CE.CryptoFailable b -> Either CryptoError b
 cryptoPassed = \case
   CE.CryptoPassed a -> Right a
   CE.CryptoFailed e -> Left $ CryptoPoly1305Error e
-
-foldlChunks :: (a -> S.ByteString -> a) -> a -> LazyByteString -> a
-foldlChunks f = go
-  where
-    go !a LB.Empty = a
-    go !a (LB.Chunk c cs) = go (f a c) cs
-{-# INLINE foldlChunks #-}
diff --git a/src/Simplex/Messaging/Encoding.hs b/src/Simplex/Messaging/Encoding.hs
index 814a536c4..846d071a1 100644
--- a/src/Simplex/Messaging/Encoding.hs
+++ b/src/Simplex/Messaging/Encoding.hs
@@ -11,6 +11,7 @@ module Simplex.Messaging.Encoding
   ( Encoding (..),
     Tail (..),
     Large (..),
+    encodeLarge,
     _smpP,
     smpEncodeList,
     smpListP,
@@ -29,6 +30,8 @@ import qualified Data.List.NonEmpty as L
 import Data.Time.Clock.System (SystemTime (..))
 import Data.Word (Word16, Word32)
 import Network.Transport.Internal (decodeWord16, decodeWord32, encodeWord16, encodeWord32)
+import Simplex.Messaging.Builder (Builder, word16BE)
+import qualified Simplex.Messaging.Builder as BB
 import Simplex.Messaging.Parsers (parseAll)
 import Simplex.Messaging.Util ((<$?>))
 
@@ -138,6 +141,10 @@ instance Encoding Large where
     Large <$> A.take len
   {-# INLINE smpP #-}
 
+encodeLarge :: Builder -> Builder
+encodeLarge s = word16BE (fromIntegral $ BB.length s) <> s
+{-# INLINE encodeLarge #-}
+
 instance Encoding SystemTime where
   smpEncode = smpEncode . systemSeconds
   {-# INLINE smpEncode #-}
@@ -174,37 +181,37 @@ instance (Encoding a, Encoding b) => Encoding (a, b) where
   {-# INLINE smpP #-}
 
 instance (Encoding a, Encoding b, Encoding c) => Encoding (a, b, c) where
-  smpEncode (a, b, c) = smpEncode a <> smpEncode b <> smpEncode c
+  smpEncode (a, b, c) = B.concat [smpEncode a, smpEncode b, smpEncode c]
   {-# INLINE smpEncode #-}
   smpP = (,,) <$> smpP <*> smpP <*> smpP
   {-# INLINE smpP #-}
 
 instance (Encoding a, Encoding b, Encoding c, Encoding d) => Encoding (a, b, c, d) where
-  smpEncode (a, b, c, d) = smpEncode a <> smpEncode b <> smpEncode c <> smpEncode d
+  smpEncode (a, b, c, d) = B.concat [smpEncode a, smpEncode b, smpEncode c, smpEncode d]
   {-# INLINE smpEncode #-}
   smpP = (,,,) <$> smpP <*> smpP <*> smpP <*> smpP
   {-# INLINE smpP #-}
 
 instance (Encoding a, Encoding b, Encoding c, Encoding d, Encoding e) => Encoding (a, b, c, d, e) where
-  smpEncode (a, b, c, d, e) = smpEncode a <> smpEncode b <> smpEncode c <> smpEncode d <> smpEncode e
+  smpEncode (a, b, c, d, e) = B.concat [smpEncode a, smpEncode b, smpEncode c, smpEncode d, smpEncode e]
   {-# INLINE smpEncode #-}
   smpP = (,,,,) <$> smpP <*> smpP <*> smpP <*> smpP <*> smpP
   {-# INLINE smpP #-}
 
 instance (Encoding a, Encoding b, Encoding c, Encoding d, Encoding e, Encoding f) => Encoding (a, b, c, d, e, f) where
-  smpEncode (a, b, c, d, e, f) = smpEncode a <> smpEncode b <> smpEncode c <> smpEncode d <> smpEncode e <> smpEncode f
+  smpEncode (a, b, c, d, e, f) = B.concat [smpEncode a, smpEncode b, smpEncode c, smpEncode d, smpEncode e, smpEncode f]
   {-# INLINE smpEncode #-}
   smpP = (,,,,,) <$> smpP <*> smpP <*> smpP <*> smpP <*> smpP <*> smpP
   {-# INLINE smpP #-}
 
 instance (Encoding a, Encoding b, Encoding c, Encoding d, Encoding e, Encoding f, Encoding g) => Encoding (a, b, c, d, e, f, g) where
-  smpEncode (a, b, c, d, e, f, g) = smpEncode a <> smpEncode b <> smpEncode c <> smpEncode d <> smpEncode e <> smpEncode f <> smpEncode g
+  smpEncode (a, b, c, d, e, f, g) = B.concat [smpEncode a, smpEncode b, smpEncode c, smpEncode d, smpEncode e, smpEncode f, smpEncode g]
   {-# INLINE smpEncode #-}
   smpP = (,,,,,,) <$> smpP <*> smpP <*> smpP <*> smpP <*> smpP <*> smpP <*> smpP
   {-# INLINE smpP #-}
 
 instance (Encoding a, Encoding b, Encoding c, Encoding d, Encoding e, Encoding f, Encoding g, Encoding h) => Encoding (a, b, c, d, e, f, g, h) where
-  smpEncode (a, b, c, d, e, f, g, h) = smpEncode a <> smpEncode b <> smpEncode c <> smpEncode d <> smpEncode e <> smpEncode f <> smpEncode g <> smpEncode h
+  smpEncode (a, b, c, d, e, f, g, h) = B.concat [smpEncode a, smpEncode b, smpEncode c, smpEncode d, smpEncode e, smpEncode f, smpEncode g, smpEncode h]
   {-# INLINE smpEncode #-}
   smpP = (,,,,,,,) <$> smpP <*> smpP <*> smpP <*> smpP <*> smpP <*> smpP <*> smpP <*> smpP
   {-# INLINE smpP #-}
diff --git a/src/Simplex/Messaging/Protocol.hs b/src/Simplex/Messaging/Protocol.hs
index e65385cba..539d6d348 100644
--- a/src/Simplex/Messaging/Protocol.hs
+++ b/src/Simplex/Messaging/Protocol.hs
@@ -146,6 +146,7 @@ module Simplex.Messaging.Protocol
     tEncode,
     tEncodeBatch,
     batchTransmissions,
+    batchTransmissions',
 
     -- * exports for tests
     CommandTag (..),
@@ -162,6 +163,8 @@ import Data.Attoparsec.ByteString.Char8 (Parser, (<?>))
 import qualified Data.Attoparsec.ByteString.Char8 as A
 import Data.ByteString.Char8 (ByteString)
 import qualified Data.ByteString.Char8 as B
+import qualified Data.ByteString.Lazy.Char8 as LB
+import qualified Data.ByteString.Lazy.Internal as LB
 import Data.Char (isPrint, isSpace)
 import Data.Constraint (Dict (..))
 import Data.Functor (($>))
@@ -174,6 +177,8 @@ import Data.Time.Clock.System (SystemTime (..))
 import Data.Type.Equality
 import GHC.TypeLits (ErrorMessage (..), TypeError, type (+))
 import Network.Socket (HostName, ServiceName)
+import Simplex.Messaging.Builder (Builder, char8, lazyByteString)
+import qualified Simplex.Messaging.Builder as BB
 import qualified Simplex.Messaging.Crypto as C
 import Simplex.Messaging.Encoding
 import Simplex.Messaging.Encoding.String
@@ -1286,13 +1291,13 @@ instance Encoding CommandError where
 tPut :: Transport c => THandle c -> Maybe Int -> NonEmpty SentRawTransmission -> IO [Either TransportError ()]
 tPut th delay_ = fmap concat . mapM tPutBatch . batchTransmissions (batch th) (blockSize th)
   where
-    tPutBatch :: TransportBatch -> IO [Either TransportError ()]
+    tPutBatch :: TransportBatch () -> IO [Either TransportError ()]
     tPutBatch = \case
-      TBLargeTransmission -> [Left TELargeMsg] <$ putStrLn "tPut error: large message"
-      TBTransmissions n s -> replicate n <$> (tPutLog th (tEncodeBatch n s) <* mapM_ threadDelay delay_)
-      TBTransmission s -> (: []) <$> tPutLog th s
+      TBLargeTransmission _ -> [Left TELargeMsg] <$ putStrLn "tPut error: large message"
+      TBTransmissions s n _ -> replicate n <$> (tPutLog th (tEncodeBatch n s) <* mapM_ threadDelay delay_)
+      TBTransmission s _ -> (: []) <$> tPutLog th s
 
-tPutLog :: Transport c => THandle c -> ByteString -> IO (Either TransportError ())
+tPutLog :: Transport c => THandle c -> Builder -> IO (Either TransportError ())
 tPutLog th s = do
   r <- tPutBlock th s
   case r of
@@ -1300,43 +1305,43 @@ tPutLog th s = do
     _ -> pure ()
   pure r
 
--- ByteString does not include length byte, it is added by tEncodeBatch
-data TransportBatch = TBTransmissions Int ByteString | TBTransmission ByteString | TBLargeTransmission
+-- Builder in TBTransmissions does not include byte with transmissions count, it is added by tEncodeBatch
+data TransportBatch r = TBTransmissions Builder Int [r] | TBTransmission Builder r | TBLargeTransmission r
+
+batchTransmissions :: Bool -> Int -> NonEmpty SentRawTransmission -> [TransportBatch ()]
+batchTransmissions batch bSize = batchTransmissions' batch bSize . L.map (,())
 
 -- | encodes and batches transmissions into blocks,
-batchTransmissions :: Bool -> Int -> NonEmpty SentRawTransmission -> [TransportBatch]
-batchTransmissions batch bSize
-  | batch = reverse . mkBatch [] . L.map tEncode
-  | otherwise = map (mkBatch1 . tEncode) . L.toList
+batchTransmissions' :: forall r. Bool -> Int -> NonEmpty (SentRawTransmission, r) -> [TransportBatch r]
+batchTransmissions' batch bSize
+  | batch = addBatch . foldr addTransmission ([], mempty, 0, [])
+  | otherwise = map mkBatch1 . L.toList
   where
-    mkBatch :: [TransportBatch] -> NonEmpty ByteString -> [TransportBatch]
-    mkBatch rs ts =
-      let (n, s, ts_) = encodeBatch 0 "" ts
-          r = if n == 0 then TBLargeTransmission else TBTransmissions n s
-          rs' = r : rs
-       in case ts_ of
-            Just ts' -> mkBatch rs' ts'
-            _ -> rs'
-    mkBatch1 :: ByteString -> TransportBatch
-    mkBatch1 s = if B.length s > bSize - 2 then TBLargeTransmission else TBTransmission s
-    encodeBatch :: Int -> ByteString -> NonEmpty ByteString -> (Int, ByteString, Maybe (NonEmpty ByteString))
-    encodeBatch n s ts@(t :| ts_)
-      | n == 255 = (n, s, Just ts)
-      | otherwise =
-          let s' = s <> smpEncode (Large t)
-              n' = n + 1
-           in if B.length s' > bSize - 3 -- one byte is reserved for the number of messages in the batch
-                then (n,s,) $ if n == 0 then L.nonEmpty ts_ else Just ts
-                else case L.nonEmpty ts_ of
-                  Just ts' -> encodeBatch n' s' ts'
-                  _ -> (n', s', Nothing)
+    mkBatch1 :: (SentRawTransmission, r) -> TransportBatch r
+    mkBatch1 (t, r)
+      -- 2 bytes are reserved for pad size
+      | BB.length s <= bSize - 2 = TBTransmission s r
+      | otherwise = TBLargeTransmission r
+      where
+        s = tEncode t
+    addTransmission :: (SentRawTransmission, r) -> ([TransportBatch r], Builder, Int, [r]) -> ([TransportBatch r], Builder, Int, [r])
+    addTransmission (t, r) acc@(bs, b, n, rs)
+      -- 3 = 2 bytes reserved for pad size + 1 for transmission count
+      | len + BB.length b <= bSize - 3 && n < 255 = (bs, s <> b, 1 + n, r : rs)
+      | len <= bSize - 3 = (addBatch acc, s, 1, [r])
+      | otherwise = (TBLargeTransmission r : addBatch acc, mempty, 0, [])
+      where
+        s = encodeLarge $ tEncode t
+        len = BB.length s
+    addBatch :: ([TransportBatch r], Builder, Int, [r]) -> [TransportBatch r]
+    addBatch (bs, b, n, rs) = if n == 0 then bs else TBTransmissions b n rs : bs
 
-tEncode :: SentRawTransmission -> ByteString
-tEncode (sig, t) = smpEncode (C.signatureBytes sig) <> t
+tEncode :: SentRawTransmission -> Builder
+tEncode (sig, t) = lazyByteString $ LB.chunk (smpEncode $ C.signatureBytes sig) (LB.fromStrict t)
 {-# INLINE tEncode #-}
 
-tEncodeBatch :: Int -> ByteString -> ByteString
-tEncodeBatch n s = lenEncode n `B.cons` s
+tEncodeBatch :: Int -> Builder -> Builder
+tEncodeBatch n s = char8 (lenEncode n) <> s
 {-# INLINE tEncodeBatch #-}
 
 encodeTransmission :: ProtocolEncoding e c => Version -> ByteString -> Transmission c -> ByteString
diff --git a/src/Simplex/Messaging/Transport.hs b/src/Simplex/Messaging/Transport.hs
index 6509c1f6f..554b9a00a 100644
--- a/src/Simplex/Messaging/Transport.hs
+++ b/src/Simplex/Messaging/Transport.hs
@@ -69,7 +69,7 @@ import Data.Bifunctor (first)
 import Data.Bitraversable (bimapM)
 import Data.ByteString.Char8 (ByteString)
 import qualified Data.ByteString.Char8 as B
-import qualified Data.ByteString.Lazy as BL
+import qualified Data.ByteString.Lazy.Char8 as LB
 import Data.Default (def)
 import Data.Functor (($>))
 import Data.Version (showVersion)
@@ -78,6 +78,7 @@ import Network.Socket
 import qualified Network.TLS as T
 import qualified Network.TLS.Extra as TE
 import qualified Paths_simplexmq as SMQ
+import Simplex.Messaging.Builder (Builder, byteString, toLazyByteString)
 import qualified Simplex.Messaging.Crypto as C
 import Simplex.Messaging.Encoding
 import Simplex.Messaging.Parsers (dropPrefix, parse, parseRead1, sumTypeJSON)
@@ -134,6 +135,9 @@ class Transport c where
   -- | Write bytes to connection
   cPut :: c -> ByteString -> IO ()
 
+  -- | Write bytes to connection
+  cPut' :: c -> LB.ByteString -> IO ()
+
   -- | Receive ByteString from connection, allowing LF or CRLF termination.
   getLn :: c -> IO ByteString
 
@@ -217,8 +221,11 @@ instance Transport TLS where
     getBuffered tlsBuffer n t_ (T.recvData tlsContext)
 
   cPut :: TLS -> ByteString -> IO ()
-  cPut TLS {tlsContext, tlsTransportConfig = TransportConfig {transportTimeout = t_}} s =
-    withTimedErr t_ . T.sendData tlsContext $ BL.fromStrict s
+  cPut cxt = cPut' cxt . LB.fromStrict
+
+  cPut' :: TLS -> LB.ByteString -> IO ()
+  cPut' TLS {tlsContext, tlsTransportConfig = TransportConfig {transportTimeout = t_}} s =
+    withTimedErr t_ $ T.sendData tlsContext s
 
   getLn :: TLS -> IO ByteString
   getLn TLS {tlsContext, tlsBuffer} = do
@@ -309,10 +316,10 @@ serializeTransportError = \case
   TEHandshake e -> "HANDSHAKE " <> bshow e
 
 -- | Pad and send block to SMP transport.
-tPutBlock :: Transport c => THandle c -> ByteString -> IO (Either TransportError ())
+tPutBlock :: Transport c => THandle c -> Builder -> IO (Either TransportError ())
 tPutBlock THandle {connection = c, blockSize} block =
-  bimapM (const $ pure TELargeMsg) (cPut c) $
-    C.pad block blockSize
+  bimapM (const $ pure TELargeMsg) (cPut' c . toLazyByteString) $
+    C.pad' block blockSize
 
 -- | Receive block from SMP transport.
 tGetBlock :: Transport c => THandle c -> IO (Either TransportError ByteString)
@@ -356,7 +363,7 @@ smpThHandle :: forall c. THandle c -> Version -> THandle c
 smpThHandle th v = (th :: THandle c) {thVersion = v, batch = v >= 4}
 
 sendHandshake :: (Transport c, Encoding smp) => THandle c -> smp -> ExceptT TransportError IO ()
-sendHandshake th = ExceptT . tPutBlock th . smpEncode
+sendHandshake th = ExceptT . tPutBlock th . byteString . smpEncode
 
 getHandshake :: (Transport c, Encoding smp) => THandle c -> ExceptT TransportError IO smp
 getHandshake th = ExceptT $ (parse smpP (TEHandshake PARSE) =<<) <$> tGetBlock th
diff --git a/src/Simplex/Messaging/Transport/WebSockets.hs b/src/Simplex/Messaging/Transport/WebSockets.hs
index ae78da1fe..486ae1f20 100644
--- a/src/Simplex/Messaging/Transport/WebSockets.hs
+++ b/src/Simplex/Messaging/Transport/WebSockets.hs
@@ -7,7 +7,7 @@ module Simplex.Messaging.Transport.WebSockets (WS (..)) where
 import qualified Control.Exception as E
 import Data.ByteString.Char8 (ByteString)
 import qualified Data.ByteString.Char8 as B
-import qualified Data.ByteString.Lazy as BL
+import qualified Data.ByteString.Lazy as LB
 import qualified Network.TLS as T
 import Network.WebSockets
 import Network.WebSockets.Stream (Stream)
@@ -72,6 +72,9 @@ instance Transport WS where
   cPut :: WS -> ByteString -> IO ()
   cPut = sendBinaryData . wsConnection
 
+  cPut' :: WS -> LB.ByteString -> IO ()
+  cPut' = sendBinaryData . wsConnection
+
   getLn :: WS -> IO ByteString
   getLn c = do
     s <- trimCR <$> receiveData (wsConnection c)
@@ -101,5 +104,5 @@ makeTLSContextStream cxt =
       (Just <$> T.recvData cxt) `E.catch` \case
         T.Error_EOF -> pure Nothing
         e -> E.throwIO e
-    writeStream :: Maybe BL.ByteString -> IO ()
+    writeStream :: Maybe LB.ByteString -> IO ()
     writeStream = maybe (closeTLS cxt) (T.sendData cxt)
diff --git a/tests/AgentTests/NotificationTests.hs b/tests/AgentTests/NotificationTests.hs
index da9f4c322..8d1ef241f 100644
--- a/tests/AgentTests/NotificationTests.hs
+++ b/tests/AgentTests/NotificationTests.hs
@@ -503,10 +503,11 @@ testNotificationsSMPRestartBatch :: Int -> ATransport -> APNSMockServer -> IO ()
 testNotificationsSMPRestartBatch n t APNSMockServer {apnsQ} = do
   a <- getSMPAgentClient' agentCfg initAgentServers2 testDB
   b <- getSMPAgentClient' agentCfg initAgentServers2 testDB2
+  threadDelay 1000000
   conns <- runServers $ do
     conns <- replicateM (n :: Int) $ makeConnection a b
     _ <- registerTestToken a "abcd" NMInstant apnsQ
-    liftIO $ threadDelay 1500000
+    liftIO $ threadDelay 5000000
     forM_ conns $ \(aliceId, bobId) -> do
       msgId <- sendMessage b aliceId (SMP.MsgFlags True) "hello"
       get b ##> ("", aliceId, SENT msgId)
@@ -572,7 +573,7 @@ testSwitchNotifications servers APNSMockServer {apnsQ} = do
 
 messageNotification :: TBQueue APNSMockRequest -> ExceptT AgentErrorType IO (C.CbNonce, ByteString)
 messageNotification apnsQ = do
-  750000 `timeout` atomically (readTBQueue apnsQ) >>= \case
+  1000000 `timeout` atomically (readTBQueue apnsQ) >>= \case
     Nothing -> error "no notification"
     Just APNSMockRequest {notification = APNSNotification {aps = APNSMutableContent {}, notificationData = Just ntfData}, sendApnsResponse} -> do
       nonce <- C.cbNonce <$> ntfData .-> "nonce"
diff --git a/tests/CoreTests/BatchingTests.hs b/tests/CoreTests/BatchingTests.hs
index fea45d4d5..c21ec8c7a 100644
--- a/tests/CoreTests/BatchingTests.hs
+++ b/tests/CoreTests/BatchingTests.hs
@@ -5,8 +5,9 @@ module CoreTests.BatchingTests (batchingTests) where
 import Control.Concurrent.STM
 import Control.Monad
 import Data.ByteString.Char8 (ByteString)
-import qualified Data.ByteString.Char8 as B
 import qualified Data.List.NonEmpty as L
+import Simplex.Messaging.Builder (Builder)
+import qualified Simplex.Messaging.Builder as BB
 import Simplex.Messaging.Client
 import qualified Simplex.Messaging.Crypto as C
 import Simplex.Messaging.Protocol
@@ -20,7 +21,7 @@ batchingTests = do
     it "should batch with 90 subscriptions per batch" testBatchSubscriptions
     it "should break on message that does not fit" testBatchWithMessage
     it "should break on large message" testBatchWithLargeMessage
-  describe "batchClientTransmissions" $ do
+  describe "batchTransmissions'" $ do
     it "should batch with 90 subscriptions per batch" testClientBatchSubscriptions
     it "should break on message that does not fit" testClientBatchWithMessage
     it "should break on large message" testClientBatchWithLargeMessage
@@ -34,8 +35,8 @@ testBatchSubscriptions = do
   length batches1 `shouldBe` 200
   let batches = batchTransmissions True smpBlockSize $ L.fromList subs
   length batches `shouldBe` 3
-  [TBTransmissions n1 s1, TBTransmissions n2 s2, TBTransmissions n3 s3] <- pure batches
-  (n1, n2, n3) `shouldBe` (90, 90, 20)
+  [TBTransmissions s1 n1 _, TBTransmissions s2 n2 _, TBTransmissions s3 n3 _] <- pure batches
+  (n1, n2, n3) `shouldBe` (20, 90, 90)
   all lenOk [s1, s2, s3] `shouldBe` True
 
 testBatchWithMessage :: IO ()
@@ -50,8 +51,8 @@ testBatchWithMessage = do
   length batches1 `shouldBe` 101
   let batches = batchTransmissions True smpBlockSize $ L.fromList cmds
   length batches `shouldBe` 2
-  [TBTransmissions n1 s1, TBTransmissions n2 s2] <- pure batches
-  (n1, n2) `shouldBe` (60, 41)
+  [TBTransmissions s1 n1 _, TBTransmissions s2 n2 _] <- pure batches
+  (n1, n2) `shouldBe` (55, 46)
   all lenOk [s1, s2] `shouldBe` True
 
 testBatchWithLargeMessage :: IO ()
@@ -69,8 +70,8 @@ testBatchWithLargeMessage = do
   length batches1' `shouldBe` 160
   let batches = batchTransmissions True smpBlockSize $ L.fromList cmds
   length batches `shouldBe` 4
-  [TBTransmissions n1 s1, TBLargeTransmission, TBTransmissions n2 s2, TBTransmissions n3 s3] <- pure batches
-  (n1, n2, n3) `shouldBe` (60, 90, 10)
+  [TBTransmissions s1 n1 _, TBLargeTransmission _, TBTransmissions s2 n2 _, TBTransmissions s3 n3 _] <- pure batches
+  (n1, n2, n3) `shouldBe` (60, 10, 90)
   all lenOk [s1, s2, s3] `shouldBe` True
 
 testClientBatchSubscriptions :: IO ()
@@ -78,13 +79,13 @@ testClientBatchSubscriptions = do
   sessId <- atomically . C.randomBytes 32 =<< C.newRandom
   client <- atomically $ clientStub sessId
   subs <- replicateM 200 $ randomSUBCmd client
-  let batches1 = batchClientTransmissions False smpBlockSize $ L.fromList subs
-  all lenOk1' batches1 `shouldBe` True
-  let batches = batchClientTransmissions True smpBlockSize $ L.fromList subs
+  let batches1 = batchTransmissions' False smpBlockSize $ L.fromList subs
+  all lenOk1 batches1 `shouldBe` True
+  let batches = batchTransmissions' True smpBlockSize $ L.fromList subs
   length batches `shouldBe` 3
-  [CBTransmissions s1 n1 rs1, CBTransmissions s2 n2 rs2, CBTransmissions s3 n3 rs3] <- pure batches
-  (n1, n2, n3) `shouldBe` (90, 90, 20)
-  (length rs1, length rs2, length rs3) `shouldBe` (90, 90, 20)
+  [TBTransmissions s1 n1 rs1, TBTransmissions s2 n2 rs2, TBTransmissions s3 n3 rs3] <- pure batches
+  (n1, n2, n3) `shouldBe` (20, 90, 90)
+  (length rs1, length rs2, length rs3) `shouldBe` (20, 90, 90)
   all lenOk [s1, s2, s3] `shouldBe` True
 
 testClientBatchWithMessage :: IO ()
@@ -95,14 +96,14 @@ testClientBatchWithMessage = do
   send <- randomSENDCmd client 8000
   subs2 <- replicateM 40 $ randomSUBCmd client
   let cmds = subs1 <> [send] <> subs2
-      batches1 = batchClientTransmissions False smpBlockSize $ L.fromList cmds
-  all lenOk1' batches1 `shouldBe` True
+      batches1 = batchTransmissions' False smpBlockSize $ L.fromList cmds
+  all lenOk1 batches1 `shouldBe` True
   length batches1 `shouldBe` 101
-  let batches = batchClientTransmissions True smpBlockSize $ L.fromList cmds
+  let batches = batchTransmissions' True smpBlockSize $ L.fromList cmds
   length batches `shouldBe` 2
-  [CBTransmissions s1 n1 rs1, CBTransmissions s2 n2 rs2] <- pure batches
-  (n1, n2) `shouldBe` (60, 41)
-  (length rs1, length rs2) `shouldBe` (60, 41)
+  [TBTransmissions s1 n1 rs1, TBTransmissions s2 n2 rs2] <- pure batches
+  (n1, n2) `shouldBe` (55, 46)
+  (length rs1, length rs2) `shouldBe` (55, 46)
   all lenOk [s1, s2] `shouldBe` True
 
 testClientBatchWithLargeMessage :: IO ()
@@ -113,26 +114,26 @@ testClientBatchWithLargeMessage = do
   send <- randomSENDCmd client 17000
   subs2 <- replicateM 100 $ randomSUBCmd client
   let cmds = subs1 <> [send] <> subs2
-      batches1 = batchClientTransmissions False smpBlockSize $ L.fromList cmds
-  all lenOk1' batches1 `shouldBe` False
+      batches1 = batchTransmissions' False smpBlockSize $ L.fromList cmds
+  all lenOk1 batches1 `shouldBe` False
   length batches1 `shouldBe` 161
   let batches1' = take 60 batches1 <> drop 61 batches1
-  all lenOk1' batches1' `shouldBe` True
+  all lenOk1 batches1' `shouldBe` True
   length batches1' `shouldBe` 160
   --
-  let batches = batchClientTransmissions True smpBlockSize $ L.fromList cmds
+  let batches = batchTransmissions' True smpBlockSize $ L.fromList cmds
   length batches `shouldBe` 4
-  [CBTransmissions s1 n1 rs1, CBLargeTransmission _, CBTransmissions s2 n2 rs2, CBTransmissions s3 n3 rs3] <- pure batches
-  (n1, n2, n3) `shouldBe` (60, 90, 10)
-  (length rs1, length rs2, length rs3) `shouldBe` (60, 90, 10)
+  [TBTransmissions s1 n1 rs1, TBLargeTransmission _, TBTransmissions s2 n2 rs2, TBTransmissions s3 n3 rs3] <- pure batches
+  (n1, n2, n3) `shouldBe` (60, 10, 90)
+  (length rs1, length rs2, length rs3) `shouldBe` (60, 10, 90)
   all lenOk [s1, s2, s3] `shouldBe` True
   --
   let cmds' = [send] <> subs1 <> subs2
-  let batches' = batchClientTransmissions True smpBlockSize $ L.fromList cmds'
+  let batches' = batchTransmissions' True smpBlockSize $ L.fromList cmds'
   length batches' `shouldBe` 3
-  [CBLargeTransmission _, CBTransmissions s1' n1' rs1', CBTransmissions s2' n2' rs2'] <- pure batches'
-  (n1', n2') `shouldBe` (90, 70)
-  (length rs1', length rs2') `shouldBe` (90, 70)
+  [TBLargeTransmission _, TBTransmissions s1' n1' rs1', TBTransmissions s2' n2' rs2'] <- pure batches'
+  (n1', n2') `shouldBe` (70, 90)
+  (length rs1', length rs2') `shouldBe` (70, 90)
   all lenOk [s1', s2'] `shouldBe` True
 
 randomSUB :: ByteString -> IO (Maybe C.ASignature, ByteString)
@@ -169,15 +170,10 @@ randomSENDCmd c len = do
   msg <- atomically $ C.randomBytes len g
   mkTransmission c (Just rpKey, sId, Cmd SSender $ SEND noMsgFlags msg)
 
-lenOk :: ByteString -> Bool
-lenOk s = 0 < B.length s && B.length s <= smpBlockSize - 2
+lenOk :: Builder -> Bool
+lenOk s = 0 < BB.length s && BB.length s <= smpBlockSize - 2
 
-lenOk1 :: TransportBatch -> Bool
+lenOk1 :: TransportBatch r -> Bool
 lenOk1 = \case
-  TBTransmission s -> lenOk s
-  _ -> False
-
-lenOk1' :: ClientBatch err msg -> Bool
-lenOk1' = \case
-  CBTransmission s _ -> lenOk s
+  TBTransmission s _ -> lenOk s
   _ -> False
diff --git a/tests/SMPAgentClient.hs b/tests/SMPAgentClient.hs
index a767218c0..c39f6fa43 100644
--- a/tests/SMPAgentClient.hs
+++ b/tests/SMPAgentClient.hs
@@ -198,8 +198,8 @@ agentCfg =
       ntfCfg = defaultClientConfig {qSize = 1, defaultTransport = (ntfTestPort, transport @TLS)},
       reconnectInterval = defaultReconnectInterval {initialInterval = 50_000},
       xftpNotifyErrsOnRetry = False,
-      ntfWorkerDelay = 1000,
-      ntfSMPWorkerDelay = 1000,
+      ntfWorkerDelay = 100,
+      ntfSMPWorkerDelay = 100,
       caCertificateFile = "tests/fixtures/ca.crt",
       privateKeyFile = "tests/fixtures/server.key",
       certificateFile = "tests/fixtures/server.crt"