I have a simple interior scene.
Render times:
20 samples
1920x1080 @ 50% 19sec
1920x1080 @ 100% 74sec
3,8 times slower.
Scales sort of quadratic, as expected.
ok
now
10 samples
3840x2160 @ 50% 38sec
3840x2160 @ 100% 380sec
10 times slower
scales “decagonic” 
One would think that the scaling should be the same. Roughly 4 times slower when doubling the resolution.
Is this a bug or not?
Depends of the size of the “tile” you’re rendering. Try changing it… Remember, usually for GPU rendering use Big tiles, and usually for CPU Rendering use Small Tiles… although also depends of the scene complexity so really no standard case here.
Thanks for the answer. I’m rendering on CPU. I know about tilesize cpu/small gpu/big debackle, but thats besides the point. A doubling in img size should quadruple the rendertime. Not 10 times longer. It gets even worse at res like 5k 6k. I do not understand this. makes it hard to produce high quality images. This is the image rendered. 1
have you check memory usage ?
have you investigated a bit further ? like getting rid of textures, no denoising, 0 bouces ect… ?
Hmmm… yup… seems strange given the image, resolution shouldn’t increase that much for the image provided. Without specs, OS, and blend file to check, i would say probably there’s memory usage issues. But also texture size and lightning processing can influence AFAIK.
Still seems weird.
Think of find out Why. the Buckets update slower with increased image size. You can test this by selecting 4K image size then press render vs the same image resolution with a render region in the Center.
Hi.
What Blender version?
If you are using a not very recent 2.92 Blender build, this is supposedly fixed in newer builds:
yep it seems to be fixed in the latest experimental build.
