I dropped the screen res so I could just about stutter around to test this but sadly no improvement when leaving that region, tried S/E/N/NN regions from the start, same issue in all regions and the wierd open a menu makes the fps back to normal until closed again.
I'm looking at the itch build as we speak and am unable to replicate the issue as of yet. I'll keep posting suggestions though in the hope that one of these things may correct whatever issue is occurring.
Do you have Vsync enabled inside your Graphics Card Dashboard/Settings? If so, could you try disabling it? I noticed that having it enabled drops the FPS noticeably for me.