System Information
Windows 10, nVidia GTX 980, driver version 372.70, all windows updates current, etc.

Blender Version
Broken: 2.77.a abf6f08 (64bit)
Worked: 2.73 (Haven't used Blender in a while so I can't comment on 2.74/5/6)

Blender is extremely laggy on my primary monitor (technically Windows considers this monitor 2, but there's no way to adjust that) It's an 27" 2560x1440 ASUS gaming monitor connected via display port. The program updates maybe once per second, not just the view port, every element of the interface lags, including the minimize window button etc in the program bar, or just dragging the Blender window around on the desktop. I normally run this monitor at 120Hz, but I've tested Blender at 144Hz and 60Hz and it makes no difference.

The most curious part about this is moving blender to a second monitor make it work absolutely fine. Even placing the Blender window 51% on the second monitor makes it work fine, but as soon as it's primarily on the main monitor, it resumes the laggy behavior.
Obviously I'd prefer to use it on my 2560x1440 display than one of my smaller ones, also ergonomically the smaller one is in an awkward spot for mouse and keyboard work. (It's a Cintiq)

Exact steps for others to reproduce the error
Just installed and tried to use it. Immediately after installing it, I loaded it up and it worked fine, the next day it's laggy. I uninstalled it and reinstalled it, same thing happened. Worked fine, then the next day it's laggy again.

I've installed 2.76 and 2.73 (or rather just ran them from the extracted zip files) and they are exhibiting this behavior as well. This is obviously some new quirk of my computer, I assume some video driver or windows update is causing this, since a year ago, 2.73 ran just fine, but Blender is the only program exhibiting this behavior, so it's obviously a combination of factors.