[Update]: There is now an even newer patch available.
While Dishonored 2 is mostly regarded as a great game, it unfortunately launched on the PC with some rather annoying technical problems. There were frequent crashes, problems with loading times, an unstable performance that can drop the framerate down to a single digit, and other such lovely little bugbears.
Bethesda and Arkane Studios are currently working on a variety of changes to combat these issues, and while the patch isn't yet available for general use, you can try out the beta version and see if it helps. As of right now the Steam beta update features a fix for sensitivity being tied to the framerate, an improvement to AMD cloth physics that could greatly increase the framerate at certain points in the game, and a fix for Dishonored 2's occasional fiddling with CPU task priority.
But first things first, if you're using an NVIDIA GPU it is recommended you update to the latest driver (375.95) as it brings with it numerous performance increases. For users with AMD cards, however, the recommended driver is still 16.10.2, though there will most likely be an update in the near future as well.
If the driver update doesn't help with your specific performance woes, make sure to try out the Steam beta patch and see if anything changes. Here's how you can install it:
• Right Click on Dishonored 2 in your Library.
• Select Properties -> Select Betas.
• A drop down menu will appear. Select BetaPatch.
• Wait a few minutes for game to update.
• When done, Dishonored 2 [BetaPatch] should appear in our Library.
• If nothing happens, simply restart Steam.
There is no release date set for Update 1.2 just yet, but according to Bethesda its going to be available at some point next week. It will contain all of the fixes I mentioned above, as well as a variety of changes to address "a broader range of issues including performance".