Extremely Low FPS when maximized windowed mode

Whenever I want to play on windowed or borderless mode if I want to play with the game taking up my whole screen I get only 30 fps. When my window is very small, I get an easy 130 fps. I am using a 4k monitor, but have the resolution in the game set to 1920x1200. I believe that the in game resolution adjuster does not work, since even if I set my resolution to the lowest option, nothing changes. When I try playing on my 2nd monitor, which isn't 4k, I get 130 fps no problem. I managed to change my resolution to 1920x1200 using geforce experience, which did give me higher frames, however when doing that the game window was quite small on the bottom left corner. I am also curious as to why it cannot stretch to fill the whole monitor, while in that resolution. 

Sidenote: I cannot even play on fullscreen on the 4k monitor, as when I change the game to fullscreen, both my monitors flash black a couple times and then stay on a black screen for a bit until they turn back on and the game is minimized. This keeps happening no matter how many times I try.