GUI is Sluggish

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http://askubuntu.com – GTK seems sluggish in general on my system; controls seem to have about a 1/4 to 1/8 second delay. This happens with both Unity (with the Ambiance theme) and Gnome 3 (with Adwaita). All controls seem to be affected, but menus are especially noticeable - first a shadow will appear, then an empty menu, then the text on the menu. Programs using GTK2 or QT do not have this problem. Gnome Shell also has poor performance, for example dragging windows in the overview is pretty laggy. I'm using Ubuntu 12.04 and my GPU is an Nvidia GeForce 8400GS (using Nvidia's driver). (HowTos)