I use a 4k screen, and minecraft runs in 4k by default but this gets me a maximum of 40 frames, even with a good gaming computer.
When I went to change the resolution of full screen to 1080p, the resolution went down, but it also wasn't true fullscreen. Only 1/4th of my screen was covered with the minecraft program, the rest was my desktop. I've seen this happen with other games, and they've fixed it.
It appears to just happen when a game isn't fully optimized to work with 4k screens.