One user reported it happening in Level 1 - Inner Hall. I've had the lag happen there, Level 1 - Entrance and Level 5 - Mines. I have trouble replicating the bug so I can't figure out what specifically caused it. In one playthrough, I would search a lizardman closest to Nosramus's lab on the eastern side of the map and the game would start lagging after I was done searching it, in a different playthough I would search the same lizardman and things would be fine. Othertimes I would enter a map and it would suddenly get very laggy, but I would return to the map later and there would be no issue.