Something else that's happened but I forgot about because since classes started I haven't had time to play long enough for it to happen anyway. It looks almost, but not quite, like a severe memory leak. I'm curious if it's happened to anyone else as well since I haven't seen it reported anywhere else even though it is serious.
Trying to make something that resembles a bug ticket

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Summary: Huge, sudden slowdown if the game is open for too long.
Priority: Up to Nadeo. The game is still playable it just needs to be restarted lots.
Description: After I've been playing for a while -- times differ for multiplayer, solo mode, local play, or editing, the game will suffer from a huge slowdown and the only way to recover it is to restart the game. Occasionally the game would still be playable even slowed-down if not for the annoying message that comes up telling me that the game could reduce my graphics settings (and if I actually press "yes" and reduce the graphics settings it doesn't affect the framerate at all) In multiplayer this is especially annoying because it happens about every track and-a-half -- meaning that the track that is playing when I join the server works fine, but about halfway through the round on the second track the slowdown will happen, ALWAYS when I press backspace to retry or, in time attack, when I finish a run and start another -- it's annoying here because I can't gain rankings on the first map that I play on a server, and I can't get to the end of the second map before I need to restart the game. So because of this bug my world ranking is stuck at some absurdly high number and my ladderpoints are stuck at 235 (I think) and have been since I started to try to play seriously even though in practise I've been doing a LOT better than that (my performance is comparable to TMUF where my ladderpoints are over 45k!), I haven't been able to gain rankings at all.
In solo play this takes a lot longer to happen -- I'd estimate about fifteen complete runs before the problem comes up -- and again, it's all of a sudden unplayable. It's not a gradual decrease in framerate like you more often see with a memory leak.
In local play it depends on the mode. On "single track" play this doesn't happen at all. In hotseat mode it usually happens before the first track is even halfway finished. I haven't been able to test splitscreen yet due to the other bug I posted.
In the editor I usually need to restart the game several times before I can test a section of track, and I always need to restart it before I record a mediatracker ghost or validate the map. In the editor and only in the editor, it tends to happen a lot faster if I minimize the game while the editor is open (I hate to say this but in the editor alone this isn't a huge issue, I've used plenty of worse editors! It's the rest of the game where it's more of a problem).
More information: In earlier game builds such as the multiplayer beta, I was able to eliminate this problem entirely except in the editor by turning bloom effects ON (yes, ON, that's not a typo). In the current game build this doesn't work to prevent the problem.