Big maps dometimes aren't a problem. I remember one of my old cs maps, de_pyramid thad occupies all the available space in the map editor and runned fine on an old athlon 1 ghz with 256 MB RAM and a 32 MB GPU, of course the height was limited to the one of the pyramid itself, but even on that old rig the performance with bots wasn't so bad. I have, in the other hand, a small map cs_sacrificio tgat runned very poor because I have used a bunch of rows of columns that made a lot of tiny faces, so Salmon is right, check for those tiny brushes and turn into func_walls whatever needed so you reduce compiling times and brushes chopping each other.