Three things:
-First the teleporter should have some nice looking effect, not only a star textured wall (every other game I know has it: Quake3 the UT series, Serious Sam, ...)
-Second, the shooting flares of the Machine Gun disappears to slow, so when you move the flare is always to far on the left/right/front/back
-no realtime level lights/shadows, instead compiled light maps like in Quake3 or UT, I think this is a great speed factor, because the game runs much faster for me when I turn them off
moHiJ