Most likely reason is that you have VSync turned on for your game. VSync (vertical synchronization) will only ever work for a game if it's in exclusive full screen (which is what you're doing by maximizing). You can change this setting by using the QualitySettings option for VSync. Taken directly from the link I've provided. This Example()
function should only ever be used when VSync option is wanted as turned off.
using UnityEngine;
using System.Collections;
public class ExampleClass : MonoBehaviour {
void Example() {
QualitySettings.vSyncCount = 0;
}
}
You can also turn it on or off in the default Quality Settings here (using menu: Edit > Project Settings > Quality).

Both pictures taken from Unity's linked pages, since Unity sometimes likes to change the current manual and deprecate links or images.
There's also one more setting that could affect target framerate. Try setting it to a high value.
using UnityEngine;
using System.Collections;
public class ExampleClass : MonoBehaviour {
void Awake() {
Application.targetFrameRate = 300;
}
}
Rendering
inCPU Usage
. It seems that a function calledGFX.WaitForPresent
causes this issue! \$\endgroup\$