On a related note, Progressive GPU+CPU rendering seems broken as it’s half the speed of GPU only.
Maybe this is an argument for making the rendering choice of GPU+CPU explicit in the Device selection in Render Properties along with GPU only and CPU only? If there are going to be cases where you want one vs. the other it would be nice to be able to set this at the .blend level rather than having to go in and out of Preferences all the time.