After updating nvidia-drivers to 295.40, I had my kde sessions very slow (several seconds to open any menu) Reverting back to 295.20-r1 solved my problem. Reproducible: Always
http://www.nvnews.net/vbulletin/showthread.php?s=2047c5f16735f9379ec7ceb2afefbf40&t=178460 So if you have one of the cards mentioned and experience the symptoms (which seems so), downgrading + waiting for a fixed nvidia-driver is the solution.
My card : 00:10.0 VGA compatible controller: nVidia Corporation C73 [GeForce 7100 / nForce 630i] (rev a2) I'll wait with 295.20-r1 installed :) Thanks !
As everyone here seems happy, I am going to close this as UPSTREAM while we wait for Nvidia to fix the issue.