Instructions for viewing the bug:

From starting place, move up 3 floors, following the up ramps. You may need to scroll the cam out or move down a little bit. On the third level up there is a troll stuck next to a door. If you play the fort, the FPS will be very low, but simply unlocking this door will allow the troll to run up and the FPS to quickly get back to a normal level.

Just before saving, I actually had citizens peacefully walk past this troll. It didn't seem to notice them or try to engage, and remains focused on the locked door.

As you can see from my nearby forbidden beast traps this bug does NOT affect beasts, though there may be other unintended behavior I have exploited with the doors... (most of them have been additionally walled off but there is at least one with the wall yet unbuilt, held back only by a door--but this causes no FPS trouble). And I actually have seen Cave Dragons destroy doors in fort mode ?? Though not on this save or version. Anyway that is all I know about the issue hopefully!

I'm guessing this fort was generated in 50.09 or so, though I am playing on the latest patch of the beta. I am using the latest DF Hack as well but no other mods or such