From Fedora Project Wiki
NVidia issues with KDE 4
Background
There are some performance issues of NVidia proprietary driver and KWin with effects enabled.
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.