I have no idea why, but it's running like utter shit on my PC (i5 4460, 16gb ram, 4gb 380), far worse than anything else I've played on it, and changing settings seems to do bugger all. It's a constant sub 30fps, even on lowest settings.
I checked the drivers and they're all up to date, there doesn't seem to be anything else running in the background, and I'm hitting a pretty consistent 60fps with no dips past 50 on Rise of the Tomb Raider at mostly high with a few very high settings, so I'm kind of flummoxed as to where the problem is.
Thankfully though it looks gorgeous and plays perfectly on my PS4, so I'll just get it on that, but if anyone has any suggestions for what the hells up with it on my PC I'd be grateful.