Gnome is a bit faster than KDE. While GTK+ 2.x is several times slower than GTK+ 1.x, it still manages to run adequately well (however Metacity is so slow when resizing an window it will redraw its button bitmaps in a really ugly fashion). KDE is plagued mostly of the fact that all its applications are slow to load. In fact, even the smallest Gnome application (e.g. calculator) is slower to load than the big and fat Blender because of the number of shared libraries is linked against! But KDE's performance on loading its apps is worse.