I have recently built my new PC (Ryzen 3700x + VEGA 64) and tried rendering BMW demo. The difference rendering speed caught my attention.
When rendering with CPU only, the time consumption never exceeds 3 minutes. When rendering with ONLY GPU (unchecking the Ryzen 3700x in preferences while keeping the VEGA on) the time is around 2 minutes.
However, when checking both the CPU and the GPU, the rendering time increased to more than 6 minutes.
According to the information gathered online, the "CPU + GPU" configuration should be faster than either CPU or GPU alone, but instead it turns out much slower. Have I done anything wrong and what should I do about it?
P.S. By inspecting the task manager the GPU does not seem to be loaded when rendering, the fan is not roaring either (it does when running games). Have no idea what this means either. Please help.