Well I managed to fix the issue, it was actually because of the lifespan of the snow particles. The lifespan was the main reason why the game slowed down out of nowhere.
It may look like I reduced it too much in this shot, but in-game the snow effect still looks pretty good:
All I had to do was change
part_type_life(snow,300,400)
to
part_type_life(snow,0,100)
No more slowdowns from this point.