[Sharlin]

A similar long-time bug in Dwarf Fortress: if you have a door or hatch marked as impassable by animals, but a stray tame critter (usually a cat) wants to get through, it will never give up trying to find a path to the other side. This can have a very noticeable effect on your fps, especially if there are several animals all trying to get past the impassable portal.

(Of course, one could argue that it's incredibly realistic behavior for a cat to very insistently demand to get past a closed door. It would be even more realistic if, once the door is finally opened, the cat would immediately change its mind and lose all interest in getting through!)