Fortnite Full screen feels a thousand times smoother than windowed fullscreen?

At full screen I have about 200 FPS and no drops and it feels a thousand times smoother. I have an AMD RX580 and just 144hz monitor and not to forget Freesync or whatever that means. I'm really not very good with PCs. Sorry ^^. But windowed full screen I have about 144 and it often drops to 90 or 80 and it feels really ka k e unsmooth. What can this be?

Because the whole other stuff has to be rendered.