Basically when you run at GDQ 8, your GPU renders the screen at resolution. Lets say, 1920x1080. When you run at GDQ 9, you are rendering at 1.5x that, and GDQ10 is 2x that. So even though you are running at 1920x1080 your GPU is doing calculations as if you were running at 2280x1620 and 3840x2160 respectively. The higher you increase your GDQ, the more load you put on your GPU exponentially because you are essentially increasing your resolution. Plus, on top of it, the GPU then has to re-size the image to your resolution, increasing the load further.

The important thing to note is that the increase in load is exponential.

And yes, centigrade.