I'm also experiencing performance issues with my Dell XPS M1330 (8400M GS). It's actually fine after start up for a while, but after working for a bit, say 2 hours, it starts to get so sluggish that scrolling through a text file in KDE 4.6 looks like a slide show, literally. A very short term fix would be switching to the virtual terminal (eg. Ctrl+Alt+F1) then back to desktop. It relieves the problem for a few minutes, then it comes back again
Does anyone know if it's a known issue, or any solution to this? I'm using nvidia 270.30 driver.