Exclusive fullscreen vs Fullscreen window FPS

Hi all,

Not quite sure where to post this so I'll try here. I am doing some simple benchmarking using FPS on a build of my game. I am targeting 1080 resolution however my monitor is 4k. I get roughly twice the FPS in "Exclusive fullscreen" vs "Fullscreen window", i.e. from about 50FPS to 100FPS. Typically in the editor I am CPU bound but I see 100% GPU utilisation when running standalone build in both display modes.

I can achieve the same performance in "Fullscreen window" if I manually turn my desktop resolution down to 1080, so it seems like the window upscaling from 1080 to 4k is halving the framerate(!) Is this expected behaviour? Can I assume end users will be able to use exclusive fullscreen when running my game?

My specs:

32GB ram
i7 6700hq
1070m
SSD

Thanks in advance for your help!