'Horrible' is probably overstating it, but it was substantially worse than I'd personally want to play. This was just after then NEXT update, so maybe it's been updated again, but it hasn't to my knowledge.
I think I tried lower resolutions, lowering the settings right down, and still was left with a pretty horrible experience. It's one thing on a controller, but another altogether when you want to use a mouse. Ymmv, of course. I think I've read analysis saying this is a game that can really benefit from an i7, more so than usual.
I enjoyed this when I first played it at launch, but then drifted off to other titles. Coming back after the NEXT update it is amazing how much new content has been added and how it changes the way you play the game. If you are looking at reviews trying to make up your mind make sure to look for those made after / with the NEXT update, to give you an idea of where the game is at now. There were a lot of negative reviews at launch due to the state of the game at the time vs expectations / hype before the release.
I'd be interested to see how it runs on your system at a slightly lower resolution. On one of my older PCs it runs at a steady 60fps at 2560x1440 (my monitor for that rig's maximum resolution). That's only got an i7-4770 @3.5ghz (turbo boost up to 3.9ghz) and a Nvidia 780 4GB GPU.
With an i5 6600k & vega 64 it should be a surprise that only a few games can do 4k / 60fps.