High CPU usage in window mode.

When running my game in a build, I’ve noticed that it takes a lot more CPU to run when in window mode.

Example:

1.I’m in the main menu of the game, in full screen.
Now the task manager will show that the game is using about 2% to 4% of the CPU.

2.Moving to window mode, bg window that takes almost the whole screen.
Now the CPU is on 10% to 20% usage.

3.Now I’m shrinking the window to a smaller size (Resolution of 660x383) (the monitor resolution is 1920x1080)
Now the CPU is on 40% to 60% usage.

I’m using unity 5.5.2f1

Anyone?..