(technical specs in my signature)
This has been biting me, intermittently, ever since Cubase 5 Mac (I am now on Cubase 6.0.2, running Mac OS X 10.6.8 ), and it is only today that I have... at last.. been able to "pin it down".
(N.B. I can still run Cubase 4.5.2 on my machine, and the problem I am about to describe does not occur there, nor does it occur in the Standalone application. Furthermore, none of the other instruments in the VCollection exhibit this problem.. it is only in Jupiter 8V (v1 and v2)..
The conditions for this slowing down of the GUI are *very specific* (I can wait more than 10 secs before it reacts to the mouse!.. but there is no problem with the instrument itself.. it still plays o.k.)..
The Jupiter 8V doesn't like Cubase 5 or Cubase 6 to have too many windows open *if they are on my 2nd screen!*.. the more windows (e.g. Mixer, Channel Settings, other plug-in GUIs) that are open on the 2nd screen, the worse it gets (it doesn't matter if the Jupiter 8V itself is on the main monitor or the 2nd, but if i drag them all back to the main screen, the problem goes away!)
I have done as much troubleshooting as I can..
completely fresh Cubase Preferences, and the Jupiter 8V being the only VST plugin in my VST plugin folders.
New empty Project.. all is fine, but if I then load my Default Project (which has Mixer, Control Room Mixer on the 2nd screen), I get this GUI slowdown.
I eventually found that I could put all my usual Preferences and VST plugins back in place, with my usual Default Project template, and the only thing I had to do was close all the windows on my 2nd monitor (or drag them over to the main screen) to get the Jupiter's GUI responsive again.
Btw, my screen resolutions..
main monitor 1600 x 1200
2nd monitor 1600 x 1080