I have a 5K iMac and I switch between macOS and Windows 10 often. I have Optifine installed since I play with shaders. My computer can't run Minecraft in 5K with shaders so as a compromise I select 1080p. 

I play Minecraft on both operating systems. In macOS, making Minecraft run in 1080p actually runs it in 1080p. However, in Windows 10, making Minecraft run in 1080p actually makes it run in 3840 by 2160, and then downscales it to 1080p, which significantly hinders performance for me.

I originally thought this was an Optifine issue, however, after testing, I found out that it was in fact, the base game. 

This happens with other resolutions as well. Running it in 720p HD makes the game run in 2560x1440 and then downscales it to 720p.