I have the exact same issue. I am playing on ROG Ultrawide 34 inch monitor with 3440 x 1440 resolution. My specs are: GTX 1080Ti, 8700k, 24gb ram, 250gb nvme ram. It runs fine on ultra, but fps drops down in the bigger cities to under 20s.
My brother has these specs: GTX 1080, 6700k, normal HDD. He seems to run ultra fine at 1080p standard resolution, and gets more FPX than me.
I would like to reduce my resolution to get some extra performance, but the aspect ratio gets screwed up. I don't even mind if the edges get clipped to give me 16:9 resolution instead with 1080.