When rendering materials with more than one or two nodes, the CPU tiles in CUDA GPU+CPU mode are extremely slow. The GPU tile finished entire image in 20 ish seconds, meanwhile, to even cancel the render it took an additional 10 seconds for the CPU tiles to finish their first sample. This means to finish those 7 starting tiles (16x16, 100 sam), the CPU running under CUDA would probably take almost an hour. The CPU on its own takes about 3 1/2 minutes for the whole image.
Is there any specific cause for the CPU being thousands of times slower in CUDA than plain CPU, or is it a bug? Doesn't seem reproducible with 'plain' materials like a single principled node, even when using volumetrics or caustics. I have a feeling the procedural textures play a large part since Eevee also bogs down very fast when you start combining them.
.blend demo attached. Running bench in Blender 2.82a on Linux with GTX 1070 and 6700k