lagging cursor from magic mouse and magic trackpad
Thanks for the hint with the radio interference, that seemed to be a large part of the problem.

In the end I did a lots of thing to get it going (consider it took like 6 hours all in all):


I switched to cable network in that room (almost). WiFi and Bluetooth definitely where interacting negatively. BTW: cable networking feels so much better!
the WiFi base-station was moved away from the Mac mini to the opposite side of the room.
the Magic Trackpad is switched off most of the time. It looks like there is some kind of interference between Magic Mouse and Magic Trackpad.
I deinstalled or updated any system preferences app, especially everything that has to do with GUI control.


Why I didn't move to a cable-mouse? I'm used to work with the swipe gesture a lot, and I'm not aware of a cable-mouse that understands Mac gestures.