If you have the latest version of GZDoom (preferrably a devbuild), then start the map and wake up the monsters by shooting. Gameplay slows down quite a bit. But it gets far worse when you open one of the two doors on either side of the exit and wake them up.
Open the console and type "profilethinkers 7", close the console and re-open it. At the bottom, one of the biggest lag producers is none other than the flares.
The culprit is easy -- disable the ACS calls for toaster checking and play around in the map again. You'll notice immediately you have much more of your FPS back. Rather amusing that but if you want an explanation for it, see the spoilers.
Spoiler: How the hell does that even work!?