We've played together a few times where the game slows down a great deal in the sewer, and it lasts for many minutes. I think it's only happened in the sewer. Also, we both lost the use of our hands sometimes. We couldn't get our wands to appear, couldn't grab fruit, and couldn't do anything with our tome. We'd keep playing, and it would magically fix itself sometimes, possibly during the boss fight. Other levels didn't seem to have this issue. This was Quest 3 standalone.
#Issues for 2 mages in the sewer
dense gulch
willow goblet
The slow is probably because of nests, those have been reported to devour performance right now