Hm, I notice that FPS takes a heavy hit when moving into populated areas, GMAPs and moving through level links on the GMAP still (as from 5.255 onwards, this doesn't happen in 5.254)
Edit: I made a short video as an example of this. Things to notice are:
1) Alone off the GMAP = Maximum 21FPS is achievable.
2) Entering the GMAP = Sharp drop before coming back up a little bit
3) Moving closer to populated areas (NPCs, Players and more noticed Projectiles) = Drops again
4) Moving back into an unpopulated area = Back to 21FPS
(Best in expanded 480p, I didn't mean for it to record my entire screen)
Direct link:
http://www.youtube.com/watch?v=6lryA8zJ9MI