The difference is in the corners of the room. And that's all. Literally. It's almost like some sort of AO.
For final compiles, -chop 32 and -texchop 16 would be reasonable enough, while you can freely increase them to 512 and 256 in non-final compiles.
Chop 512, texchop 256. Compilation time on my 2007 laptop: a minute or two. Chop 8, texchop 4. Compilation time on my 2007 laptop: an hour. An hour for just this room.