Version:            (using Devel)
OS:                Linux
Installed from:    Compiled sources

Test environment:
- Compal FL90 with nVidia GeForce 8600 GT
- 2 GB or RAM
- Archlinux x86_64
- nvidia drivers 177.82-1
- xorg-server 1.5.3-4 (1280x800 resolution)
- kde(libs) 4.2.0-2008121816 from the KDE 4.2 daily arch packages
- desktop effects on (KWin FPS, wobbly windows and few others)
- few plasmoids (Folder View listing images, 3 resource monitoring plasmoids) - http://www.linux.rk.edu.pl/site_media/resources/linux.rk.edu.pl/images/kdenV.jpg

The problem:
KWin goes max to 60 FPS. Clicking on (locked) plasmoids (resource monitors) can decrease the FPS by about 20 (where clicking doesn't make any actions for the plasmoids). Launching images from the Folder View also decrease the frame rate, sometimes hangs the desktop for few seconds.

As noted on http://techbase.kde.org/User:Lemma/KDE4-NVIDIA I've used:
nvidia-settings -a InitialPixmapPlacement=2 -a GlyphCache=1
Which made the desktop more responsive (no big FPS loss when clicking on plasmoids, lower fps loss when launching images from Folder View). InitialPixmapPlacement=1 or 3 made any action on the desktop to lagg and 1 FPS for KWin. Even with InitialPixmapPlacement=2 the CPU load can be very high as the laptop fan often goes to highest speed for short ammout of time (doesn't happen on KDE 3.5.10 etc.)

Running OpenArena test for comparison (https://bugs.launchpad.net/xserver-xorg-video-intel/+bug/195843/comments/2) I've noticed that FPS test made on KDE4 differs greatly from results got under other WMs:
GNOME - 401,5 fps, KDE 3.5.10 (kdemod): 390.2 fps, TWM - also about 400, KDE 4.2 SVN - about 235 FPS (doesn't change for InitialPixmapPlacement 1-3).