Click the arrows on the “Window Mode” option until you hit Fullscreen. If Fortnite has been running in Windowed mode, head back to the Video tab in the Settings menu.
Running games in Windowed mode tends to reduce performance and really only works well for simple 2D games, as full screen mode allocates more resources to the program. Note that the Epic Games Launcher must actually be kept open at all times, as Fortnite can’t open as a standalone program without that app running in the background. Shut down all other applications on your machine before running Fortnite, from chat programs like Discord to any other apps that may be running in the background. This is great advice whether you are running Fortnite on Windows, mobile, or Mac to ensure you’ve got the most memory and CPU cycles available for the game.
