Got unlazy and booted up the VM and I think I figured it out after a bit.
This was half true, I guess? Duration held didn’t seem to have an affect on notes getting repeated but if a note was repeated, all currently held down notes’d be repeated too.
If a key is being held down, pressing another key has a chance to also re-fire every other held down note as well. So if you’re holding down H, wait for it to fully fade out, and then press E, there’s a chance that both E and H play, even though the only new input is E. This also applies to issues where a note gets stuck down while not being physically pressed. I couldn’t get the one where notes stick when leaving the pianos range to happen, idk if that one’s fixed or if it was just choosing not to happen, the one with selecting the instrument I didn’t test because my computer sorta hard crashed when I clicked back into the VM, but I do know that the one where notes repeat and get stuck after getting off an instrument quickly does still occur, which can lead to this issue. It also will just happen if you have a single note you’re playing–it’ll double up sometimes. But the slam-dunking of performance is moreso related to every note the piano thinks is held down getting played twice, which could literally be every note if you’ve got wack rollover on your keyboard or do jank setup with the above mentioned bugs. I’ve been saying keys and notes, but same applied for drums I believe.
Still didn’t test bongos because i forgor 
I did more testing in plaza and it doesn’t seem to happen there. You can still get notes to be stuck down, although it’s different timing compared to condos, but they don’t all repeat when a new key is pressed, which probably means it wont lag people in Songbirds to hell and back (I think).
Also the guest-testing was done in Proton, so that may have some affect on how hard it was hitting. You can definitely hear in some of the clips the issue with audio crackle, but that’s more of Valves or other Proton maintainers issue.
Here’s a video of not that much going wrong in the plaza:
Here’s me testing it with the notes being stuck down from the quick-get-off thingy:
And then here’s my performance dropping so low the Steam overlay FPS counter stopped rendering for a bit:
