IIRC the major performance problems were at one point caused by all of the useless cosmetic addons you can apply to every student through special modes/commands. I don't remember the exact specifics and I'm too lazy to go looking for them, but every toggleable addon or easter egg that didn't drastically alter models was applied to everything all of the time with all of the caculations and load that requires, but were simply being hidden or made invisible. Like if you had an easter egg that gave everyone a fancy hat, even if you hadn't toggled the easter egg that hat was still there and its position and physics were being constantly updated for every NPC in range even if you couldn't see it. And if you're doing this dozens and dozens of times and not optimizing it well...
TL;DR - lots of useless calcuations being made for things not actively being used due to poor design choices, though no idea how true that is today. Unity Engine updates seem to have gotten better at parsing through garbage.