fullscreen windowed low fps when in focus

OS: Windows 10
GPU: EVGA GTX 1080 FTW
Monitor: Dell S2417DG monitors x 2

I found this post from googling my issue, and I think I found the solution for some folks experiencing this.

TL;DR unplug your second monitor and plug it back in

I bought a 2nd Dell S2417DG monitor, added it as my new primary, and setting the second S2417DG as the secondary. I started noticing the above reported issue while playing Overwatch in borderless window mode. The FPS counter in the main menu would stay at a constant 60 FPS. This is the max it ever gets on the main menu, but in game it will go to the max 165hz. However the frame rate looked choppy as if I was dropping ever other frame. When in fullscreen mode there was no problem. I also noticed that when clicking into the 2nd montior while the game was running in borderless, the framerate became smooth again.

I found several suggestions to try to resolve this -- including changing many options in Nvidia Control panel and using nvidia inspector to change settings and nothing seemed to resolve it.

THE FINAL SOLUTION after hours of frustration was to simply unplug the Displayport cable from second monitor while the game was running then plug it back in. Once the 2nd monitor disconnected, the issue went away. After plugging the second monitor in it was detected, the issue never came back.





Source: 
https://www.nvidia.com/en-us/geforce/forums/geforce-graphics-cards/5/282916/fps-drops-when-borderless-window/