That sounds weird. I played it on a 1060 3gb and an old i7 920, and it was fine. Have you got SS on in SteamVR? if so lower it to 1.0 and try again. Also, if you have SS set in the Pimax software, try lowering that to 1.0. If it still runs at low fps, then something is definitely wrong.
Lower the world and shadow details etc. But they shouldn't cause trouble on a 1080ti.
I will download it out again, to set what performance I get.
Ok, so I can get about 60fps at 100% res, when I set world detail and shadows to low, or close to that if I only set one of them to low, and it looks pretty good.
What is your cpu model?