I have this problem too on Android 11. Is this only happening when your phone screen is off for about 30 seconds and you are running on battery? This is what happens to me on Xiaomi mi8 using Pixel Experience 11 ROM. My ADB logs say something about “deep freezing all processes”. I tried everything (unlimited data, disabling doze via ADB, disabling battery optimization) but it did not help against the freeze. As soon as I wake the screen, Syncthing is able to do about 25 Megabytes/sec. data throughput again over WiFi. Its obviously my Android ROM or system restricting Syncthing. As soon as I plug in the charger, Syncthing also does its job well when the phone’s screen is off.