I, too, had some pretty bad input lag in borderless and slightly less in windowed, but still there. What I ended up doing was turning all of the graphics settings down to low and changing the render scale to 100% (some settings weren't actually set to low with the preset so you may have to change them manually). I also turned off "limit FPS" and changed my resolution to as low as possible. After that, in windowed I couldn't distinguish any delay and in borderless there may be a delay, but it's very small.
For reference, I'm rocking a 4GHz octacore and a 980ti. Dunno why I had any delay in the first place, but it wasn't because my machine was lacking power.