Its a highly customized UE4 engine (demo is 4.27).
https://steamcommunity.com/app/1013140/discussions/0/6266251726404689484/#c6266251726406624589 If anyone can solve the stutters, they can as they have access to the engine in a way many devs don't. If they do or not is another question.
I'm guessing the low fps you get is on the stutter and not elsewhere? If its elsewhere then I'm not seeing that and not sure why you're seeing it. I went all over the map and its fine. Conversly, the stutters are fast enough that I'm not "counting" the fps there and they would obviously be low as they are hitches/stutters. Some attempts at a workaround can make it feel worse.
Still, I can't wait. Trying to avoid playing the demo further is terribly difficult lol. Curious to see if the demo gets locked out tomorrow. (Edit - looks like it may be staying up past the Steam event. But, I'm working on playing other things now until the game arrives.)
Edit - PS5 also has a bit of hitching/stuttering (demo dropped there too now). Possibly 90% better overall compared to what I see on PC. The one demo area doesn't really have any. In the wide open areas of the large map traversal is where more can be seen including one quick one while spinning the cam at the start. They're all very minor though imo but am sure someone may still complain. I've seen this in other well known games on the PS5 in graphics mode. Don't know if the demo has a performance mode since I don't ever use it but didn't see it in the the game's settings and I'd assume therefore it doesn't have it (although never know if only the full game will).