No its because of the way the level is designed its very hard to cull the geometry without making it obvious. Its a dense vertical stack where you can actually see down to the swamp at the bottom of the map, enemies and all.
but but but Dark Souls doesn't respect my time as an adulting gamer!!
Anyway I thought the bad performance of that area was that even if you are on the top section of Blightown the game still renders the venom pit and its interactions on the bottom.