Hello!
I have the same issue on my setup. I have setup my screen layout to have the World Settings and the Details on the other monitor, just a tiny little window, but as soon as I have it up, sometimes it drops my FPS around 40 frames and sometimes it completely goes down to ~20 fps. As soon as I minimize the window on the other screen the fps goes back up again.
Comp specs:
GTX 980Ti, Intel i7-4790, 16 gb’s of ram, running off of an SSD.
Both monitors are hooked up the same graphics card and through DisplayPort.
Cheers,
Shad
// EDIT: Seems to be an issue with Nvidia GeforceExperience, I went to the Settings tab, under General and disabled NGE In-Game Overlay and the issue seemed to go away.