I am using Blender 2.79 on Ubuntu 16.04.2. I have gtx610M(closed source drivers installed) and Intel(R) Core(TM) i5-3210M CPU 2.50GHz processor
I have a very simple scene. A cube placed on a plane.
I've rendered this scene with 128 samples. 32x32 tiles. With CPU I got it rendered in 25.56 seconds. But when I switch it to GPU I get it rendered in 1 minutes and 7 seconds. I tried increasing the tile size to 256x256 and then I got it in 54 seconds.
Can that be natural? 1 minutes of GPU rendering time for this scene is obviously larger than normal, how about the CPU? Is 25 seconds ideal?