I also have a problem when I put it on performance mode, everything goes smoothly, but when I start a game, my PC crashes completely, black screen, impossible to get out, I have to restart the PC with the button. I've tried everything; do you have any suggestions about what it could be?
Click on Start.
Type %localappdata%, then press Enter.
Go to \FortniteGame\Saved\Config\WindowsClient\.
Right-click on GameUserSettings.ini, then click on Properties.
Uncheck the Read-only box.
Click on Apply.
Launch Fortnite.