Cycles GPU+CPU Optimization: If a faster (GPU) core is idle, it takes over a slower (CPU) one.

A general render speed for each core is tracked. If a render core is significantly faster at rendering tiles than the others, and if it is idle, allow it to take over for slow ones. (This also covers cases where the GPU is slower than the CPU. )

Discussed some previously here: blender.community/c/rightclickselect/CWbbbc/