So this isn't technically a bug with my game at all (more on that below) but it is, none the less, a technical issue which is negatively effecting development, and to which a simple browser search presented no solution (or even any particularly relevant results.)
My problem is basically...exactly what the title says. I woke up today, and found that my computer had restarted in the middle of the night. Nothing terribly unusual about that, but after booting up Game Maker Studio 1.4, making a few minor tweaks to a level I'd been working on last night, and compiling the game for a test, something..."funny" happened. I almost immediately noticed that the timing and speed of everything was off. The game seemed to be running at around 70% of its normal speed. Regardless of any circumstances, as well; every room, every screen, however packed or empty, was effected by this same unusual torpor. I even created a new room with nothing but the player character and an invisible floor; just as slow as any other part of the game. To completely rule out the possibility that it could be any changes I made, intentionally or otherwise, I downloaded an executable demo build that had worked fine on the day I uploaded it. Sure enough, slow as molasses.
For further testing, I played a few games in my Steam library, as well as a Studio 2 project by one of my friends. Both were absolutely fine, and I can also confirm that my game is running fine for that same friend. It's not a particularly demanding game, either; pixel art, fairly low resolution, an old-school lighting engine based on surfaces and gradient sprites, and simple post-processing shaders to adjust bloom, saturation, and contrast. Even with both the lighting and shaders disabled, the game ran no faster.
I'm barely even looking for solutions at this point, I'd be happy for a few good theories, or even just affirmation that I'm not literally the only person this has ever happened to. My only real guess is that a Windows 10 update screwed me over, somehow, for some reason; but that's obviously not much to go on.