Does anybody knows how to fix this? I have only the mods that makes the trees and flora look taller, and a few minor ones, including the Oblivion Character Overhaul.

The game when Im outside, with all the trees and bushes and grass runs superb, when Im inside too. But anytime I face a single NPC it drops my FPS. I tested this even when the NPC is in a room, and I'm not phisically seeing it, but I know he is just across the door or the wall, it still kills my FPS. Help?