On my Metaquest 2 the fps will drop from 120 to 36 after a few minutes und will stay there.  A restart of the app does not change anything. Only after a restart of the Metaquest ist starts again with 120 fps. Any idea what the problem is?  

It's possible the headset is overheating and thermal throttling. What is your resolution in the advanced video settings, and your autofocus accuracy in the user settings? Maybe try lowering those values to see if it helps. Setting sharpness to 0 will also help with performance. Also, charging your headset while you're using the app will increase the thermal load, so it increases the chance of the headset thermal throttling. I haven't run into this issue myself, so it might vary from headset to headset.

Thanks for the answer. I have always a powerbank connected to the headset. I will try without it and see if this solves the problem.