Bethesda has updated the Dishonored 2 launch FAQ with tips to help people that are experiencing performance issues. Most of the recommendations are of the “turn X setting down” variety and the company does say that 1440p is only for high end systems running GeForce GTX 1070 or GTX 1080 cards. No word on a timeline for a patch.

General recommendations
Avoid using Alt-Tab when playing. Relaunch the game if you see performance dropping after using Alt-tab.
Avoid having background applications running in addition to the game.
Video Settings
If you have performance issues, try lowering the resolution.
Use 1440p resolution only if you have a very high-end GPU ( GTX 1070/1080 or equivalent)
Keep the V-Sync activated, especially if you have large framerate fluctuations.
If you have framerate under 30 fps, adjust the “Adaptive Resolution” between minimum (50%) and default value (75%).
Advanced Settings
Use the “Auto” presets:  this should adjust your visual settings without impacting your framerate.
If you still have performance issues with “Auto” presets:
Try deactivating the “TXAA Anti-Aliasing”
Try lowering the texture details

Ongoing Discussion

Source: [H]ardOCP – Bethesda Responds To Dishonored 2 PC Issues