Don’t do that! Use Fullscreen only, this will greatly increase the performance as your computer will have an easier time handling the game. Some use windowed full-screen mode for faster tabbing out and in of the game. Watch Twitch streams, VODs, clips and download them for offline viewing.
this ultra low latency mode is made for gpu heavy games when your fps is under 100fps. Ultra-Low Latency Mode to lower the input lag.