you need at least a 970 and even that is cutting it close, i have these specs and have to play on medium settings

Ryzen 5 1600 3.2Ghz 6 cores
16 gigs DDR4 ram
RX 580 4 gig
windows 10

I should probably also mention that i'm playing it in windowed mode cause of how much fps loss I get in fullscreen.

My old PC had a GTX 760 with a i5 can'trememberwhat and 16 gigs of RAM, it ran fine on medium settings during the early alpha stages in which the game was nowhere near optimized. My new PC is running on 1080p max settings manually capped at 119 fps with g-sync. I'm pretty sure it's your CPU, an i5 with 3 digits is ancient and bottlenecks the hell out of your graphics card (which in my opinion should be sufficient to play at least on low settings). RAM isn't a big issue either, a lot of modern games show little to no difference in performance between 8 and 16 gigs.

Windowed mode and full screen have little to do with your frame rate.

Nothing you can really do other that get a better PC and/or set everything to the lowest settings.