Why is GPU rendering is so slow compared to the CPU? I recently bought a new configuration to increase rendering performance with Cycles. It's  an Intel Xeon E5-2620V3 6 core 2.40 GHz with 32GB of DDR4 ram. Video card is Nvidia Quadro M4000 8GB GDDR5. I assumed that GPU rendering should be faster than CPU rendering on this machine, but the opposite is true. For a rather simple scene (1000 samples), CPU rendering took about 18 minutes whereas rendering it with GPU took one hour and 23 minutes ... (which is even way slower than when rendering it with GPU on a lower performance computer). Blender does recognise the card (it shows up in the system settings) but does not really seems to use it, I guess. Anyone any idea what I might be doing wrong here or why the GPU rendering is so slow?
thanks,
Dominique Adriaens
Both your CPU and your GPU are quite good. Have a look at blenchmark. Your CPU is right up there and comparable to good GPUs. So it's not to be expected that the difference in render times will be huge. However, to optimise render times for CPU/GPU you should use the auto-tile add-on from user preferences. Just activate this addon and render times will be optimised.