From Fedora Project Wiki
NVidia issues with KDE 4
Background
There are some performance issues of NVidia proprietary driver and KWin with effects enabled.
Issues
KWin uses too much CPU
KWin uses too much CPU with NVidia proprietary drivers (about 75%) which is quite non-sense for window manager. Seems to be fixed in KDE 4.1.1 and latest NVidia drivers from Livna. Now the consumption of KWin is somewhere around 4%. If it's not better then try:
QT_NO_GLIB=1 kwin --replace
See http://labs.trolltech.com/blogs/2008/08/22/rant-desktop-effects-never-more/ for more information.
Overall KDE performance is bad
How-to
Tweak NVidia settings.
nvidia-settings -a InitialPixmapPlacement=2 -a GlyphCache=1
Edit your xorg.conf and add these options:
Option "AllowSHMPixmaps" "0" Option "PixmapCacheSize" "1000000"
KWIN_NVIDIA_HACK is enabled by default in KDE 4.0.3.