I had a similar problem. I have an I5-8400 and a 1080 (yes I realize with a 1080 I really should have a I7 but I figured since games aren’t heavily multithreaded and the 8400 has a decent boostclock I’d be fine, with my budget at the time I could either get a I7 and a 1070 or a i5 and a 1080) So far every game iv’e played I’ve had no problem getting stable 60fps at ultra, except KCD However when I set KCD to ultra I had dips below 30! So I tried going down to High and I still had regular dips to 45. I ended up having to go down to medium just to get a stable 60fps. So I opened up taskmanager, afterburner, and core temp to figure out what was going on and it turned out my gpu was managing fine but my cpu was never dropping below 85%-90% usage and most of the time it was at 100%! Honestly I don’t think I’ve ever seen a game this cpu heavy. once again this is the only game I can’t run at 60fps ultra.
Edit: Okay I was mistaken about a couple things (I haden’t played the game in a while) I tried playing it on the ultra high preset in Rattay (without the “HD” expansions) I did have drops as low at 25fps but I realized I could run it fine on high (not medium like I remembered) and also I discovered that although my cpu was at 80% my ram was also at 80%, so it was probably a mix of both problems, (I only have 8 gb of ram I built my PC during the shortage) it’s still surprising and I swear my cpu was at 100% last time I checked, even though it didn’t go that high this time. So I guess it’s a mix of this game loving cpu power and loving its ram.