Dishonored 2 PC port problems are real.
I have a decent, mid-range desktop (GTX 980, 16 gig ram, AMD FX 6350 processor) and I was getting 20-35 FPS on a mixture of low and very low settings. I played around with the Nvidia settings abit, and I'm better now, 40ish FPS on middle settings, but it's still quite choppy and with some graphical tearing.
I also have to run it in windowed mode, or the game crashes on the main loading screen. Which takes hideously long to load, btw.
All in all, it's a real shame, because otherwise the game is good fun to play. I'm doing a Corvo low chaos playthrough, and though I'm skipping some of the levels a bit fast (to progress the story), and I'm liking it. Enemies seem a bit smarter, a bit more attuned to their environment. Verticality is definitely a thing. And the non-lethal options are pretty awesome...you no longer have to be a master of stealth to keep your hands clean. Clockwork Mansion mission was a bit messy, but otherwise I've had no problem only neutralizing the target and getting away clean (a bunch of guards following me through a rewired wall of light at the start of the mission did not help).