For some reason Cycles GPU rendering speed increases enormously when using bigger image sizes.
I am now doing 6000x3000px image and it takes almost three second to calculate one sample. And it is not the case all the time. When using smaller images (like 1920x1080px) rendering is as fast as expected. Also, sometimes rendering starts quite fast, but then slows down to one sample at second speed.
And I know that the doubling of pixels means four times more calculation. The Increase is now more than ten times. I might be from 5 minutes to 8 hours.
I am using 640x640 tile size but I have experienced with different tile sizes too. Using bigger tile size than that is not possible because blender crashes (which is odd thing too of course).
I use the newest build of Blender 2.8 but I have experienced same issue with 2.7. My first thought has been that it is because of the memory issue, but why image size would have effect on that? Blender is able to download whole scene to GPU memory and memory peak is not more than maximum of 5 gigabytes.
I made a thread about same issue 7 moths ago, but at that time I wasn't up to date about the problem: Cycles GPU rendering turns to really slow with big image sizes.
Someone had the same kind of issue here (https://blenderartists.org/t/cycles-gpu-render-speed-drops-extremely-when-increasing-render-size/638059), but the solution of closing T-panel in the UV/Image Editor sounds really odd and didn´t work in my case.
Does anyone one what might be the case?
Specs: Windows 10, Inter Core i5-7600 @3.5GHz, 16 GB RAM, NVIDIA GeForce GTX 1070