I was doing some test renders on a project at 5% resolution and I realize it takes much less time than expected to 100% with the same render settings.
100% resolution takes about 7 minutes to render a single frame, while 5% takes little over 6 seconds. And I'm wondering, is there some sort of ratio to calculate render times depending on the resolution of the render?
It is hard to predict. Render time is divided on two pieces: a preparation (resolution independent) and an actual rendering (depends on resolution mostly). Second part, typically in quadratic dependence on size. If you make the image 50% instead of 100%, it will make render time of this part 4 times faster. This is because the render image is 2-dimentional, you decrease the image size in both sides X and Y in the same time, so that 50% of render size means 25% of its area