I was about to post something similar, I'm using an RX570 (8GB vRam, 16GB sys ram) and whether I have everything on ultra or on low, it does not change, it sits around 25fps to 28fps. I know the settings are taking effect because I can see the change in lighting etc, but theres something not right. I know I'm not CPU bound either. The only option that changes the FPS is intensity, and that goes from 120fps (lowest) to 8fps (highest) but lower settings look pretty bad. I forced off PostProcessing and other tweaks via the console, and still sats at 28fps, 100% GPU load.
At 1080p on other unreal engine based games, (whether that be commercial game like Ark, or similar demos like bioasshard) usually sit on 60fps vsync capped with some headroom to spare.
Performance a side, demo looked interesting. When on the ground and you approach a locker, the camera gets confused and goes inside the character and some animations bugged out where the faceless was misaligned and put arms though her stomach and pounded the air, but I expect such things so early on in development.