if ur using grass models in ur level you should merge more than one grass model together so you have a patch of grass rather than one grass model. or...you could go into display00.wdl if ur using a6templates...and change the fps maximum. change the max entities as well. also use the orbit cameera so when you look at ur player from a top view the level runs fast regardless of where you are or how many modles are in the area. change time_smoothing to .99 which is the max. split your world into different parts and put portals at points where parts end... when you walk through the portal everything behind you disapears and things infront of you appear allowing you to,A, make your world bigger and,B, make more stuff. but the grass tecnique is the best thing to lower the fps rate of an "outside" level. heres an example of a grasspatch model i made to aid you.
http://www.savefile.com/files/9010495 
