I'm having the same problem as you. My setup is:
CPU: Pentium G3258
GPU: GTX 1050 Ti
8GB RAM
I know I'm coming in at the opposite end of the performance spectrum (this isn't my main rig, and it's kind of a Frankenstein), but according to Fortnite I should be able to run 1080p 60fps at mid-high settings.
I've had the same issue as you, CPU at 100%, GPU around 50%. Game freezes often, but when it's not frozen it runs very smoothly. I also have an issue where textures won't load right away (and I assume trying to load a massive amount of textures at the same time mid-game contributes to the freezing). Do you have the texture problem as well?
After searching for a while online I found that the problem is most likely Fortnite, as many players are experiencing freezing, not-yet loaded textures, and various other bugs with the new updates. Epic Games said they are working on it. There are hundreds of YouTube videos showing different supposed fixes from streamers and others. None of them worked for me but maybe they will work for you.
Here is one that looks promising. I didn't try it because I was afraid my setup didn't have the overhead performance to prioritize the game, but yours looks like it does. It's also potentially using regedit (if you get the accessed denied pop-up) so I would be extra careful.
View: https://www.youtube.com/watch?v=OjFVyJ1Xr-E
Long post short it looks like we might have to wait for the Fortnite team to release a patch, but I would also really appreciate an answer that we can do right now, especially hardware related as I am looking to upgrade this rig. Sorry I couldn't help more than that.