I think I understand the taproot (not implemented) performance behavior. Now that it's in that range the CPU has started to drop off the peg, but SSD remains maxed out. In other words, CPU and SSD are pretty well balanced in terms of being the limiting factor (both at 100%) so as CPU demand falls, rate remains constant. IOW the times on the mini are likely reasonable without the implementation. But that also implies the mini might not benefit much from CPU optimization, apart from not needing priority and/or as many configured threads.
(edited)