Originally Posted by MrAli
please do that ...
Well ok, it seems like "nomodset" does not make a huge difference, but uninstalling nouveau helped a lot.
However, KDE is still quite slow sometimes, just from switching windows and desktops there are noticeable lags and the frame rate (which you can measure using a "desktop effect") goes from 60fps to 30fps.
These slow-downs don't occur in razor-qt, however it has other problems (such as missing wifi app), thus I'll also give KDE 4.8 a try or go back to KDE 4.6 until these problems are fixed.