Fullscreen mode makes CPU usage go significantly up - even on an empty scene ! - when running in a display monitor of 3440x1440.

[koodikulma-fi]

Godot version:
3.0.6 (stable.official.8314054)

OS/device including version:
Win 10, Intel i7-7700K @ 4.20GHz, NVidia GTX 1080, Monitor 3440x1440

Issue description:

• When running godot in full screen mode, the CPU usage goes very high (at least for some monitors). For example, running a near empty scene (only one node in it) in fullscreen, makes my CPU usage go up to 15-20%..! When running this scene in windowed mode (even if maxed), the CPU usage is at 2% (with the same background apps running).
• This happens only when the game is running in my main display (monitor), whose resolution is 3440x1440. When I run the game fullscreen in an extra monitor (a pen display) whose reso is 1920x1080, CPU usage stays nice and low.

Extra information:

• The problem happens only after the fullscreen is toggled on, and goes away after it's toggled off (while running the game).
• When was testing this with a game (a 2D side scroller), tested that the FPS stayed at 60.
• It doesn't seem to matter what the project resolution is set at. For the game I had it 1920x1080 and the window/display/stretch/mode was "2d" and aspect "keep", but as I said, this happens right out of the box as well (with "ignore" and "disabled").
• For the empty scene it made no difference whether V-Sync was enabled or not. However, in my game, when I had VSync disabled in NVidia's Control Panel, the CPU usage did NOT go up when entering fullscreen. (Makes no sense to me at all why disabling vsync makes this problem disappear in a complex scene, but not in a trivial one.)
• The behaviour seems to be the same whether running in editor or as stand alone.
• Haven't had any similar fullscreen / windowed issues with any other game played on this computer.

Steps to reproduce:

• Open Task Manager or other performance monitor for testing purposes.
• Create a new Godot project.
• Create an new scene and put a Node in it and save the scene.
• Run the scene in a display monitor whose size is 3440x1440 (very possibly others, too). Watch CPU usage (mine is about 2% with Godot editor and some background apps running). Change window size, no problem.
• Open project settings and set display/window/size/fullscreen to true. Save and run the scene again, and watch CPU usage (mine stays steadily at about 15% or 20% with same background apps running).

Minimal reproduction project:
Note. The core issue seems to be about the display monitor resolution. This is just a near empty project with fullscreen toggled on.
FullscreenProblem.zip

[KoBeWi]

Looks still valid in 3.2.2 rc3

In 4dede9e the CPU usage seems to be high both on fullscreen and windowed, so not sure if there's difference.

Tested on Windows 10 GTX 1060

[Calinou]

We need someone to run Godot with a profiler to see why it's using so much CPU. You can use one of the debug builds here.

[Marigem]

Same thing happens to me.
Same CPU as the author, Win10, GTX 1050TI, 1080p monitor.
CPU usage goes significantly up when Vsync is enabled and the game is in fullscreen mode. If Vsync is disabled or the game isn't in fullscreen mode, then CPU usage is very low as it should be.