Sorry to ask, but I noticed my plotting PC is producing less plots every day since the difficulty is rising. From once 40 Plots a day down to 20 Plots a day.
Does the difficulty have an effect on the plotting speed? Some people said no, so I am not sure.
Is the bottleneck the NVMes or is there more stress on the single thread CPU process with rising difficulty?
Difficulty has nothing to do with plotting. Something must have changed in your plotting setup or where the plots are being stored in their final destination. That is quite the drop in plots per day. Mine has been staying steady at about 36 plots/day.