Has Fortnite been on your PC since the update this morning? I have a 1080Ti and a Ryzen 7 3800x, before the update to Epic, I always had around 150 FPS now only 40 and on low settings I always had 250-350 now only 120.
Nope by no means
Update the driver of the graphics card?!
The driver is up-to-date (thanks anyway for your answer)