Originally Posted by Plagman
Also, are you using Firefox 3? Is there any particular website that reproduces the problem?
A performance problem when displaying certain images in Firefox 3 with InitialPixmapPlacement 2 that caused symptoms similar to what you described has been fixed earlier (the rendering would hitch when scrolling to the bottom of digg.com, amongst other things). The fix will be available in a future driver release.
It is well known that nVidia has horrible 2D performance on Linux. See here http://www.larsen-b.com/Article/290.html
It is unfortunate that I had purchased my card uninformed of this. Do you know when the 2D performance of the nVidia cards will be fixed? I have an 8800GT.