Poor performance in maximized or windowed mode

When running windowed mode, the framerate is unplayable (stuck @20-40fps) while on fullscreen mode it runs perfectly fine at 144fps when vsync-limited, without vsync it goes up to 400-900fps.

• Pre-1:

Same world, same settings, same everything:
Before pressing f11: https://i.imgur.com/NJWq6dP.jpg 
After pressing f11: https://i.imgur.com/OcKHVNx.jpg

Worth mentioning that after pressing f11 again (back to windowed mode) performance is alot better than it was before (~100fps) but then it only lasts about 5-20 seconds then it goes back down to poor performance.

Tried the same on an empty world (void preset) and i had rock solid 12fps on windowed, fullscreen made it 800, back to windowed 144fps, and after 5-20 seconds to 50, then slowly down back to 12.

Shift+f3 when on windowed says display_update.unspecified takes 80-85% of the resources, while on fullscreen it's 5-10%

all it says is root.display_update.unspecified is using up 83%

 

• 1.13 Release:

Added video as of 1.13 release: https://www.youtube.com/watch?v=iRSExpJ1yoA

The video freezes when i fullscreen (F11), but you can see in the bottom left graph how fps were much higher while fullscreened when i go back to windowed