All of those; wide open areas mean the engine has to draw more polygons, and a bunch of dynamic entities in one area mean they have to constantly check that the player hasn't done something with them. If you're going to have big levels, put lots of corridors/small rooms in. Computers will fair better.
And Harry, I don't know about that. FPSC doesn't run slow on all computers, especially mine. I still can't make 40*40 areas, for obvious reasons, but I can have pretty big areas without to much trouble (I don't have a top-of-the-line PC, either). Also, if you thought
this is bad, you should have seen the EA
.
EDIT: Didn't see your post, Storm. It took me 3 mins to write that
!
Click above for Project Luna demo! Resident Evil Remake is teh fergin' best!