Hi! Just experienced the same exact issue yesterday with GMS1.4. Last I checked like a few days back, my games were fine, but yesterday afternoon, every one of my games on Game Maker Studio was running like 1/4 slower than usual. This was after a Windows update.
have you discovered if the 'Speed margin' really puts the games' speed back to normal again and for ALL MACHINES, so as not to only be the correct speed when playtesting, and then having them run at, like, double speed when a friend tests on their computer?
The origin of the issue is still a mystery, I assume?