Alright, I figured it out.
Gamemakers sequences, sprite animations and whatnot are calculated based on the games room_speed, not deltatime.
If you set the room_speed to 9999, but the game is running at 800 fps, then the game is technically lagging, and you want to know what happens when a game lags? It slows down. Because gamemakers sequences play based on the games room_speed, it plays very slowly.
My solution to this for now is to set the room_speed to fps_real.