General update post:
Through some slightly less specific searching, I found out about the "sleep margin" global game setting. People seem to agree that the default value of 1/ms should always be changed to 10/ms anyway, and doing that...completely fixed everything, no changes to any computer settings required. That begs the question of why I only just now needed to change it, and why only I had any such issue in the first place, but if nothing else, I can at least playtest my game again.
Since I'm still pretty ignorant on the matter though, another question; have I fixed this issue, or is it now going to play at hyper-speed on literally anyone else's machine? To my paltry understanding, it has more to do with frame calculations than actual game speed, but you can never be too safe.