I've said this elsewhere; this happens in any game mode where they have restricted the classes you can play and those classes contain Party Time variants.
What appears to happen is that because most people don't have the Party variants, and they get unlocked for these particular game modes, they jump at the chance to play the Party Imp and Party Corn. What seems to happen is that the party variants have a lot of particle and light effects associated with them, with all the confetti, flash and glowing going on and if you happen to be in a portion of the map where the amount of all of these effects gets saturated your machine essentially chokes and the frame rate crashes.
Players who weren't forced to try to render the over-saturation of confetti, flash and glowing keep playing and those choking on the rendering get stalled out. It's the only explanation which fits why it only happens on the Super Mix events which limit character selection while simultaneously opening up Party variants temporarily to those who don't own them already.
When, and if, people ease up on the Party Corns and Party Imps the game should become playable again, of if you manage to keep your distance from the chaos.
