I thought at first of overlays like Nvidia's Share tools, but disabling it didn't help. I don't have anything else running that would interfere.

I tried running the game with a FPS overlay, and it's constant 20/21 FPS in the gameworld (I think that's what it's capped at) When I enter a menu like Inventory it drops to 7-8 and as low as 3-4

The strange thing is that for example, the more items there is in a particular character's inventory, the lower the framerate - so an empty inventory would be 8 FPS, and one with 10 items is 4 FPS. Basically, the more stuff there is to click, the worse it is.

The framerate issue doesn't persist when you exit the menu to the gameworld - but comes back when I go back in.

It makes me suspect some sort of memory/CPU cycle error during data collection of what can be interacted with in the menus. I'm not an expert though.