I don't know why, but after playing for four hours the game shit the bed. 5 FPS (changing settings doesn't affect anything), and changing to windowed/etc doesn't work (still image of confirmation screen). Verified game three times and it's still the same. Drivers are updated so that can't be the issue, so I'm completely baffled. First game that's ever acted out like this. Probably going to reinstall.

i5-6600K
RX 480 8gb
16gb ram

Reinstalling hasn't fixed anything. Not sure how to proceed.

I assume you have tried reaching out to support? Both GoG and to Ninja Theory?

Are you by any chance in a big area with water nearby? Because I have a similar problem - 17 FPS on lowest settings, lowest resoution and nothing seems to fix that. My GPU isn't at full load there either. I have similar issues with PUBG which also happens to run on UE4 and I too have a Polaris AMD card (RX580 running Radeon Software Version
17.7.2 drivers) - so it might have something to do with that.

Running an nVidia 1050Ti and in that scene a stable 60 fps on max settings at 1920*1200...

Yeah, sounds like it's a problem with Unreal and AMD drivers. I hope that gets fixed soon. I just suffered through this area and after it it's back to a solid 60 FPS, even at higher settings - so if you think you can do it just try to plough through. Just see it as a gameplay element - yet another trial! Okay thanks, that's actually good to know - sounds like it's just not optimised for AMD cards.

Ok I haven't used an AMD card in ages, not because they are bad but because they are largely unsupported.

But first, verify if you have the latest drivers (those updates are generally game spécific), if you're good on that front
Go on their forums (if they still have) and see if others have the same issue, if not mail their support

At the same time open a ticket with ninja theory, link this forum. Oh and also did you think to look in the steam forums?
There is more users, so maybe more answers (That helped me for witcher 2)