I have an RX 570/ i5 4690k. The AMD cards are great but I had narrowed many of my problems down to the GPU's texture filtering. It's worked on several games... Set up a profile for your game on your GPU, default all the settings, set the texture filtering to standard or performance. High Quality tends to cause some fps problems in some games.
It may not be the AMD's filtering operation but the textures in the game. It's not as good as NIVIDIA's.
That sounds off, but it has worked for me, in PUBG. PUBG is terribly optimized, you know. I put texture filtering to performance and it ran fine.
Other than that check your power options , make sure everythings 100% high performance.
You're not going to be able to do 120fps in most games of the last few gens on a 580. That card isn't that powerful, you should aim for 60. I actually play on 30 quite often... I usually play on Ultra, 2x SSAA 1080p or 4k at 30fps, gpu won't break a sweat.
I have dyslexia, you're on a iMac, Jesus Christ. jk That should run Fortnite like butter actually. It may be thermal throttling. If iMacs have bios go in there and turn off turbo boost for certain and see if thermal throttling is optional. 90'C is the maximum temp you can get.
If you want to get radical, crack it up and put new thermal paste on the chips. I have to do that once a year or so, it really works.