When I first reached the surface (where you first encounter assassins) the game dropped down to about 5 fps when looking towards the center of the level, maybe 20 fps looking away. I noticed the assassins were being very passive and seemed more interested in jumping around randomly than attacking me. I finally managed to kill one, and instantly my framerate jumped up to 40+. The other times I encountered assassins, my framerate dropped a bit but nothing like that.
Has anyone else encountered a similar problem?