Hi Andreas, I see you have an nVidia card. There seems to be a problem with Radeon drivers and whatever method is being used to generate/update the GUI. I switched to an nVidia card a few months ago (for reasons not related to this problem), and upon reading your post decided to check again: I can now have any number of instances open without any slowdown whatsoever.

Oh, yes, I did a test run on a small project. 10 tracks with two instances of Monlith each, another 7 tracks with one Monolith each for mixing, and finally three buses and the master track with Monolith on each one. No problems as long as I don't have more than one Monolith window open.