Slow Alt Tab

When I Alt Tab in Fortnite game, it takes like 5 seconds to alt tab, and sometimes it crashes, is there anyway to fix it? I tried on GTA V, it alt tabs instantly, I played both on Fullscreen. also I have a 144hz 1080p monitor. Windows 10 Pro 1709

Soon enough I'm buying a new PC and I found the NZXT S340 really nice. But just a while ago, the Corsair 100R came out and i have only found one good review (not too bad), so here I am asking ye for help. Thanks very much ! Oh, and by the way, the S340 is about €68 and the 100R is €49...