My Android app talks to my own ECG device. Everything runs perfectly fine when using an old (now discontinued) Kyocera E6560 phone that was running Android 5.5. I've recently purchased 8 different phones, running more recent versions of Android. All the new phones transfer SPP data 7 times slower than my old Kyocera phone. I can't find any new phones that can run as fast as the E6560.

I've narrowed down the speed decrease to the read() function. When I create a socket like:

bluetoothSocket = bluetoothDevice.createRfcommSocketToServiceRecord(UUID_SPP);
bluetoothSocket.connect();
inputStream = bluetoothSocket.getInputStream();

then the following line takes about 7 times longer with newer phones:

byte[] data = new byte[1024];
int dataSize = inputStream.read(data);

Any ideas why this is or what further steps I can take in debugging this?