Sorry, this was absolute nonsense. I forgot about BT latency: I just changed my setup and started rehearsing over a small digital mixer including a BT receiver.
There is no such delay between optical and acoustic metronome if I use the internal speakers of my tablet (unhearable when playing) or USB audio.
There is no such delay between optical and acoustic metronome if I use the internal speakers of my tablet (unhearable when playing) or USB audio.
2x Samsung Galaxy Tab S5e with Android 11 / UI One 3.1 - AirTurn BT-200