About reducing LAG a bit more:
-reduce decals to a certain "max" number
- Also I saw in some maps there are fogs or fumes coming out the ground and when it was on my sight the game lags, maybe fixing the sprites to make less lag would be better...
-Use monster spawn points(I think it's already been done): zombies only spawn when you pass some area, for example, instead of when starting the game zombies spawn all throghout the map, they just spawn where you are(obviously in the spawnpoint, not in front of you)

uh,they dont spawn till you walk near em,so thats not a problem,spawning is fine,but i find d3d mode to boost a good 40 fps,with minimal eye candy loss

MkilBride is correct.

stole my sig!!!!!!!!!!!!!!!!!!!