I am running Kubuntu 10.04, updated from 9.10.
In 9.10, KDE 4.3 did not run stable with effects / compiz enabled on my system, I guess not KDE's fault. However with effects disabled, the GUI feels very slow. Nearly every GUI interaction has an annoying delay, be it scrolling, app switching, navigating in menues etc.
It feels like if it were running inside a virtual machine. Only Firefox behaves better, seems to do rendering a different way.

With desktop effects / compiz enabled, this delay disappears. I will now test again if it got stable with the update, but I don't like to do much such testing on this machine as I'm working on it.

I've a GeForce 7600 GT, using nvidia driver. Screen resolution is 1680x1050.

I would appreciate any hint about what I could check.

look under "style" in the appearence section of systemsettings. It has a fine tuning tab that lets you adjust cpu usage, and also if you configure oxygen you can turn off some of the fading effects.