To test this out I just setup 31 thread CPU Slot on a 5950x (with a 3070 that was already running a GPU slot) running Linux Mint to see what happens. In Linux if you run nvidia-smi from a terminal it tells you the GPU utilization on the right hand side. I went from 99% utilization when running GPU only to bouncing around between 92 and 98% once I added the 31 thread CPU slot. My time per fold on the GPU slot dropped from 1 minutes, 16 seconds per fold, to 1 minutes, 21 seconds, and the PPD on the GPU slot dropped from 5,012,961 to 4,696,496. The CPU slot is doing 518,434 PPD, so it is still a net gain vs running the GPU alone, but it's definitely starving the GPU somewhat. I'm going to remove the CPU slot and go back to just Folding on the 3070.
I'm sure there are a lot of factors such as what project type the GPU Is running, what project type the CPU slot gets, etc. as far as how much it affects it, but that was the results on my quick test. In addition, if it was running BOINC on the CPU threads and not Folding@home, depending what project, etc. it may provide different results too.