I can confirm that this is a huge thing. The ONLY maps where it seems to happen is in Red Ribbon's HQ and Red Ribbon's dungeons, commonly reffered to as RP1,2, etc.
The game plays just fine up until the point when you enter those places and the lag spike is insane. You can test that by typing in the chat and there'll be no delays on it. Another way to confirm that is those places affecting it is if you leave the map and try to cast a few buffs or do a few skills on someone and you'll notice zero delay/lag, then go back into the dungeon and the lag is there again. Is a very noticeable difference.