ios: move caching images to background thread, dont use main thread scheduler for marking items read (#4840)

apps/ios/Shared/Views/Chat/ChatView.swift

@@ -797,9 +797,10 @@ struct ChatView: View {
         }
         
         private func waitToMarkRead(_ op: @Sendable @escaping () async -> Void) {
-            DispatchQueue.main.asyncAfter(deadline: .now() + 0.6) {
+            Task {
+                _ = try? await Task.sleep(nanoseconds: 600_000000)
                 if m.chatId == chat.chatInfo.id {
-                    Task(operation: op)
+                    await op()
                 }
             }
         }

apps/ios/SimpleXChat/ImageUtils.swift

@@ -391,7 +391,9 @@ public func imageFromBase64(_ base64Encoded: String?) -> UIImage? {
             return img
         } else if let data = Data(base64Encoded: dropImagePrefix(base64Encoded)),
             let img = UIImage(data: data) {
-            imageCache.setObject(img, forKey: base64Encoded as NSString)
+            imageCacheQueue.async {
+                imageCache.setObject(img, forKey: base64Encoded as NSString)
+            }
             return img
         } else {
             return nil
@@ -401,6 +403,8 @@ public func imageFromBase64(_ base64Encoded: String?) -> UIImage? {
     }
 }
 
+private let imageCacheQueue = DispatchQueue.global(qos: .background)
+
 private var imageCache: NSCache<NSString, UIImage> = {
     var cache = NSCache<NSString, UIImage>()
     cache.countLimit = 1000