Fortnite is Insanely Unoptimized (especially after Chapter 1 ended, and they added Next-gen graphics, it just got worse!), not totally your PCs fault...
things to do:
- Set It's priority to High, sometimes it doesn't utilize all cores
- if you're on 60hz monitor, or 120, Lock fps to your refresh rate (even if you don't use vsync), it seriously helps with stutters for some reason
- Turn off any post-processing effect, e.g sharpening (it helps a tiny bit), also set textures to high, lowering it can cause stutters in my experience
lastly, play around with Multithreaded rendering, for a 4c4t cpu it should help, but might also not, so try switching it, hope it helps, first one is what I've seen to...