Thanks for looking into it (again). The tailing also occurs with the preference window of Renoise and while I move the window the spectrum analyzer slows down. Maybe that’s another problem and related to this:
All other installed programs I have behave normal, i.e. no tailing. But I get the tailing when e.g. moving windows of Firefox ontop the Renoise GUI. This happens on my Desktop PC and my Laptop (Lenovo T440p) both running MX Linux