You could set PulseAudio to use a lower quality mode, as it is quite CPU expensive.
Furthermore, you could use a Window Manager like Enlightment, which uses less hardware power, or even a different desktop environment like XFCE.
The graphic card (driver) seems to be quite slow, with my graphics card (NVIDIA GeForce 8600M GT, a whole different class) I get with Compiz enabled
13710 frames in 5.0 seconds = 2741.572 FPS
14046 frames in 5.0 seconds = 2808.962 FPS
Metacity instead yields
25991 frames in 5.0 seconds = 5198.191 FPS
25820 frames in 5.0 seconds = 5163.993 FPS
If possible, try to upgrade some of your hardware, that's more effective than to tamper with small software optimizations.
But if you like to go the difficult way, there's a whole bunch of tweaks for ioQuake3 based games