Just because your CPU isn't maxed out at 100% all the time doesn't mean it isn't still being overloaded. You may be single-core bound and not limited overall. Look at task manager and switch it from overall view to where you can view the individual cpu cores and threads. You may find 1 core is being overloaded to 100% while the others aren't loaded much. You're probably being single-core limited with that CPU.
I happen to have a CPU older than yours and not even an i7, just a 3570K but overclocked @ 5.0 Ghz and a 1080 Ti and I can easily maintain 60-80 FPS at all times everywhere in Satisfactory even on the highest possible maximum graphics settings at 1080p, even in the middle of my factory with all the machines going. This wasn't possible last month. I used to be jerky, laggy and having hard pauses/stalls in Satisfactory, but now it's nice and smooth for me, other than autosaves.
The game is a lot better and runs perfectly fine now, if we have a fast enough CPU/System. If you do have an "I7-4790" and not "4790K" then the 4790 non-K chip is a locked chip that can't be overclocked and runs at only 3.8 Ghz max.