I can't quite figure this one out.

Any time I'm in a small, enclosed area, my FPS will drop from 60 down to mid-40's. There could be no one else around, and it still drops. I could be facing a wall, and it still drops.

Yet, when I wander around a city or any place that actually has lots of NPCs/PCs and gives my PC some actual work to do, it's a smooth 60 FPS.

It's slightly annoying considering it should be the reverse, if anything.

Ex:
http://i.imgur.com/DwQtGxj.jpg = 45 FPS
http://i.imgur.com/putfZiX.jpg = 60 FPS

Any ideas?