The pdp8 limitation back then was not the CPU but the fact that it's timing was interlocked with the core. Take core out of the timing picture with fast Sram and crank it up then things like bus timing and ripple carry are a factor. So a turbo PDP-8 is achieveable and if it were all on a chip your "8/Z" would be scary fast with current tech. However if you reduce the number of transistors as MIPS and other RISC machines early on did your dependent on raw speed at the die level and the cycle repeats. That is you end up pushing for ever higher speeds to make up for simplicity and those transistors left are hitting the speed/power wall. Then of course you can try to push that by making the machine wider. The process does the lather, rinse and repeat with new process slipping in to help out but we are approaching the sub atomic level were transistors perform badly as there are not enough molecules in them! So we go back and add more parallel cpus on the same die and that means more transistors and we go around again. Allison