Actually it is not completely true, that a desktop using only the traditional 2D accelerated drawing engines of the GPU is as snappy as the new compositing desktops used by Mac OS X, KDE 4 and Vista. Because there is one main difference: Backing Store (or whatever it is called outside of X11). The 2D accelerated variants rarely use it! With the 3D accelerated compositing desktops you basically get it for free, since every window is rendered into a texture. So whenever you move around a window, the underlying, exposed windows do not have to do complex redraw operations, just a simple texture fetch or blt is in order. This IS possible with 2D accelerated cards, but as I said, it is hardly ever done, for several reasons: video memory management probably being the biggest one, An expert on the implementation of this in X.org or similar could probably tell you more.