Well, it depends on the processor you have. The builds only improve OSL rendering, which is not the case if you use a GPU. In my test, it didn’t make a big difference with or without CPU. The problem is, when the GPU becomes so fast, your CPU may remove a part of the job and at first make it faster, but then the GPU has to wait for the latest CPU tiles to finish rendering, which kills the whole render time.
And it consumes much more energy, creates more heat and noise. So I just leave it on GPU only.
Already, with Vega64+1080Ti together ( I took a patch from brecht to get OpenCL and CUDA working together), and those speedups, it’s so damn fast that the BVH build time is often the biggest part of rendering.
I could get the CPU rendering 20% faster by making the builds with LLVM if customers ask.