diff --git a/examples/companion_radio/main.cpp b/examples/companion_radio/main.cpp
index 6b421550..d6da46f4 100644
--- a/examples/companion_radio/main.cpp
+++ b/examples/companion_radio/main.cpp
@@ -218,7 +218,14 @@ void setup() {
 // add usb interface
 #if defined(ENABLE_USB_INTERFACE)
   usb_serial_interface.begin(Serial);
+  // keep frames intact and pace the contact stream when the host is slow
+  usb_serial_interface.enableFlowControl(true);
 #if defined(ESP32) && defined(ARDUINO_USB_MODE) && ARDUINO_USB_MODE == 1
+  // a 256 byte TX buffer overflows during a contact sync, and write() blocks
+  // up to tx_timeout_ms per call against a stalled host (same reasoning as
+  // the kiss_modem tuning)
+  Serial.setTxBufferSize(4096);
+  Serial.setTxTimeoutMs(5);
   // The ESP32 USB-Serial-JTAG peripheral (HWCDC) has no DTR concept at all:
   // (bool)Serial only tells us the host has enumerated the device, which is
   // already true when the cable is plugged into a powered port. Fall back to
diff --git a/src/helpers/ArduinoSerialInterface.cpp b/src/helpers/ArduinoSerialInterface.cpp
index 317e8395..079508df 100644
--- a/src/helpers/ArduinoSerialInterface.cpp
+++ b/src/helpers/ArduinoSerialInterface.cpp
@@ -19,6 +19,11 @@ bool ArduinoSerialInterface::isConnected() const {
 }
 
 bool ArduinoSerialInterface::isWriteBusy() const {
+  if (_flow_ctl && isConnected()) {
+    return const_cast<Stream*>(_serial)->availableForWrite() < (int)(MAX_FRAME_SIZE + 3);
+  }
+  // while nobody drains the port the TX buffer stays full, so never report
+  // busy in that case: it would stall the paced streams on all interfaces
   return false;
 }
 
@@ -27,6 +32,17 @@ size_t ArduinoSerialInterface::writeFrame(const uint8_t src[], size_t len) {
     // frame is too big!
     return 0;
   }
+  if (_flow_ctl) {
+    if (!isConnected()) {
+      return len;   // nobody is listening, drop instead of filling the TX buffer
+    }
+    if (_serial->availableForWrite() < (int)(len + 3)) {
+      // a short write would tear the length prefixed framing, and as there is
+      // neither a checksum nor a resync marker the receiver would stay out of
+      // sync forever - so drop the whole frame instead
+      return 0;
+    }
+  }
 
   uint8_t hdr[3];
   hdr[0] = '>';
diff --git a/src/helpers/ArduinoSerialInterface.h b/src/helpers/ArduinoSerialInterface.h
index ecc05976..ca6d9f69 100644
--- a/src/helpers/ArduinoSerialInterface.h
+++ b/src/helpers/ArduinoSerialInterface.h
@@ -10,6 +10,7 @@ public:
 
 private:
   bool _isEnabled;
+  bool _flow_ctl;
   uint8_t _state;
   uint16_t _frame_len;
   uint16_t rx_len;
@@ -19,7 +20,7 @@ private:
   uint8_t rx_buf[MAX_FRAME_SIZE];
 
 public:
-  ArduinoSerialInterface() { _isEnabled = false; _state = 0; _last_frame_ms = 0; _conn_check = NULL; }
+  ArduinoSerialInterface() { _isEnabled = false; _flow_ctl = false; _state = 0; _last_frame_ms = 0; _conn_check = NULL; }
 
   void begin(Stream& serial) {
     _serial = &serial;
@@ -36,6 +37,12 @@ public:
   // Useful as an activity-based connection check where no DTR state exists.
   uint32_t getLastFrameMillis() const { return _last_frame_ms; }
 
+  // Optional: only hand a frame to the stream when it fits into the TX buffer
+  // as a whole, and report busy while it does not, so bulk streams get paced.
+  // Only enable this for streams which really implement availableForWrite()
+  // (USB-CDC does, the Print default returns 0).
+  void enableFlowControl(bool enable) { _flow_ctl = enable; }
+
   // BaseSerialInterface methods
   void enable() override;
   void disable() override;