What about common events with a parallel process trigger that might be running? If the performance is just bad on that one map, and it's on the whole map all the time, it's almost got to be a parallel process event or a plugin.
Or it could be something else you're running on your pc at the same time. A gamedev I knew once spent several hours programming a new part of the game, and when he sat back to test play, he turned on some music to listen to in the background. The game stuttered terribly. He then spent more time undoing his changes piece by piece, testing after each one, with no improvement. Finally deciding it needed his undivided attention, he turned off his music to get down to work, and the game ran flawlessly.