IIRC the proprietary driver has a performance issue with GTK+ themes that use gradients (e.g. Clearlooks), so try switching to another GTK+ theme e.g. ClearlookClassic or Nodoka.
Also you might want to give the open source driver, nouveau a shot; I've been having some performance issues with scrolling in Firefox, e.g. scrolling web sites that use heavy box-shadow would be a bit jerky, switching to nouveau fixed the issue; also scrolling web pages in e.g. Launchpad.net would be slow/jerky https://bugzilla.mozilla.org/show_bug.cgi?id=563720
, nouveau doesn't seem to suffer from that problem.
FWIW, the main thing I miss with using nouveau is VDPAU (ftp://download.nvidia.com/XFree86/vd...tml/index.html
), but then again a) I don't have that many HD videos and b) my CPU isn't that new but can cope with playing videos with the Xv video output.
Also the OpenGL situation has improved a lot with nouvea and latest Mesa, enabling e.g. KDE4 3D desktop effects doesn't cause any big issues (I don't use desktop effects usually, but in the ~5hours I enabled them I didn't notice any major issues with either the proprietary driver or nouveau).
(So, basically all I am saying is give the nouveau driver a shot it could improve things (or make them worse in which case the proprietary driver won't look so bad)), note that you'll have to uninstall the nvidia proprietary driver completely before switching to nouveau.