Ok. Thanks for the reply.
I can say that my pi was running with that hifiberry board flawlessly for years with volumio 2.x, but after updating to 3.x I wanted to change the system for reasons.
So I don't think it's too slow for the hifiberry amp.
What worked so far was the connection to KODI(libreelec) which I see in spotify when I go to the airplay options. But the connection seems not that good, but Ok.
If I connect via librespot it gets choppy.
So it might has something to do with the plug-in.