Well, basically, the game runs slowly with any kind of heavy sector, and by "heavy" I mean sectors with lots of stuff in them. For an example I can barely manage 10 FPS in the Caketown community build sector on Cake Build Server, which has a lot of clutter (entities) and logic in it. Planets, cluttered sectors, and other stuff like that still have a notable drop in FPS, even though the devs are trying to optimize the game, it never seems enough.