I just did a fresh install of Fedora 15 x86_64 with KDE.
KDE is mostly unusable in Fedora 15 while it was fine in Fedora 14 (with nvidia driver).
The display seems to freeze for a few seconds every few seconds.
Menus take seconds to open.
Windows leave garbage on the display when they are moved or re-sized.
New windows are often filled with garbage before they are drawn all the way.
KDE automatically disables the desktop effects.
I installed and switched to fluxbox and everything is snappy.
Without all of the KDE decoration firefox went from unusable to normal under fluxbox.
I have not installed any software not in Fedora's repository (no nvidia drivers).
System details that might be usefull:
lenovo S10 Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz 4 GB of DDR3 1067 MHz RAM nVidia Corporation G86 [Quadro NVS 290] nouveau driver is loaded and in /etc/X11/xorg.conf Screen spans two 1600x1200 CRTs