There's a BSP process that handles things like the basic player collision files as well as AI collision and navigation (the AAS files). Also, you can run a -ShadowOpt 2 compile which optimizes the shadows that are cast by lights (they're polgyons that add to your tri count; another reason why large overlapping lights can be bad), but the lights still function fully in real-time.
Larger maps can still take a while just due to the number of faces and volumes (AAS again) that have to be calculated, but it's still a very very quick process. Minutes at most.