I have had the same problem, I think ever since I switched to the KDE 4 series. Also using the nvidia driver:
> dlocate -l "^ii\s+nvidia" Desired=Unknown/Install/Remove/Purge/Hold | Status=Not/Installed/Config-files/Unpacked/Failed-config/Half-installed |/ Err?=(none)/Hold/Reinst-required/X=both-problems (Status,Err: uppercase=bad) ||/ Name Version Description +++-=================================-=================================-==================================================================================== ii nvidia-alternative 280.13-1 allows the selection of NVIDIA as GLX provider ii nvidia-cg-toolkit 2.1.0017.deb1+nmu3 NVIDIA Cg Toolkit Installer ii nvidia-glx 280.13-1 NVIDIA metapackage ii nvidia-installer-cleanup 20110729+1 Cleanup after driver installation with the nvidia-installer ii nvidia-kernel-2.6.38-2-amd64 270.41.06-1+2.6.38-5 NVIDIA binary kernel module for Linux 2.6.38-2-amd64 ii nvidia-kernel-2.6.39-2-amd64 275.09.07-1+2.6.39-2 NVIDIA binary kernel module for Linux 2.6.39-2-amd64 ii nvidia-kernel-3.0.0-1-amd64 275.21-1+3.0.0-1 NVIDIA binary kernel module for Linux 3.0.0-1-amd64 ii nvidia-kernel-common 20110729+1 NVIDIA binary kernel module support files ii nvidia-kernel-dkms 280.13-1 NVIDIA binary kernel module DKMS source ii nvidia-kernel-source 280.13-1 NVIDIA binary kernel module source ii nvidia-settings 280.13-1 Tool for configuring the NVIDIA graphics driver ii nvidia-support 20110729+1 NVIDIA binary graphics driver support files ii nvidia-vdpau-driver 280.13-1 NVIDIA vdpau driver Disabling compositing (Alt+Shift+F12) is a workaround as mentioned, and sometimes necessary when running off a battery. When compositing is enabled, kwin sometimes behaves well, but sometimes seems to eat as many CPU cycles as it can get (on a single core). Switching desktops away from a desktop containing a konsole window may be a trigger (I'm not really sure if it's konsole related; certainly I get a short spike in CPU usage following this and I seem to remember it triggering unlimited CPU usage in the past). Many times I've seen this happen with CPU usage never seeming to reduce again (unless I disabled compositing or killed kwin); I thought I saw the same thing just now but CPU load did reduce (after maybe a minute or so). The kde task manager doesn't notice the high CPU usage, but other programs like top do. Possibly related upstream reports: https://bugs.kde.org/show_bug.cgi?id=279944 https://bugs.kde.org/show_bug.cgi?id=242979 Those aren't the only reports matching "kwin CPU", but the most relevent.
