I am using vegetation to generate a lot of trees, and I have noticed when collision is turned on for them, the framerate drops significantly (about 60 to 20 FPS), but only when actually colliding with a tree (like walking into one). It continues to stay low until I stop colliding with the tree. I'm assuming this is a bug and isn't just a limitation of the engine, since it should only processing one collision.
There does seem to be some framerate drop when colliding with other objects in the world as well, so it may have to do with how much vegetation is being shown. It's strange that a single collision would drop the framerate from 60 to around 20 though.