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The default used to be 10, so if you load an old project it will just keep whatever was there.
After the game is done rendering a frame, it sleeps for a couple of milliseconds to wait for the next frame. What the sleep margin does is that it is a configurable length of time where the runner will sit in a tight loop waiting for the next frame to start - so setting higher values can help a game sync up properly with the screen and have more consistent fps. However, setting it too high will also increase the cpu used on idle by a lot, therefore you should choose a value that best fits your game.