Although the testing is purely subjective and anecdotal, back to back installations of compiz-fusion (under Gnome) with Xgl and again with AIGLX indicate that with at least the current driver from nVidia, there's more overhead with Xgl thnat AIGLX. On a desktop system with a 2.4GHz P4 and 1G memory, and an nVidia GeForce 6200 video card, OpenGL-based screensaver hacks ran poorly, with objects moving in bursts or jerks. Block tube, for example, didn't spin smoothly, and the flying toasters lurched along in their flight paths. Using either gears or aquarium in the cube brought the system almost completely to its knees, particularly the aquarium (even with limited numbers of fish). With AIGLX, however, sceensaver hacks move smoothly, even with gears spinning away inside the cube. A full-on aquarium population is still a challenge of sorts but I'll live without that somehow...