Re: 8600M GT+Compiz=Slow Animations and Black Window Bug
It's possible (and it sounds like) you are seeing the effects of the video card frequency scaling. You could verify this by opening a terminal and letting the machine idle for a minute or two and then typing "nvidia-settings -q all | grep Current" (note the values) and then typing "nvidia-settings" and then "nvidia-settings -q all | grep Current" again. If the values of the first and last numbers differ then this is probably what you are seeing (the middle nvidia-settings is just to produce enough spam in the terminal to force the card to jump to the highest frequency).
On many of the 7 series mobile cards you can disable the frequency scaling (it's VERY annoying with a composited desktop) but, I'm not sure about the 8 series cards. You can search for PerfLevelSrc on these forums for more information but, I don't know if the "hack" will work on an 8600GT.
Hope that helps.