I tried installing the game on an old iMac 333 with 160 MB RAM and 6 MB VRAM, and after some tweeking, I found some good commands to improve the performance:
r_picmic 10
r_picmip 10
r_drawstaticdecals 0
r_farplane_nofog 1
Type it in in the console. This will make the game butt ugly, but somewhat playable on older unsupported machines.