I have a similar issue, my framerate gets progressively worse the longer the game is open. I can get a steady 60 fps even in busy areas, but it degrades down into the low 40s and I can only fix it by closing the game and restarting it.

edit: Just to be clear, it's not caused by more people entering the area over time. The game will be running in the low 40s, then I can close it, immediately re-open it and log back in, and the exact same area will be locked 60 fps.