With 2021 knowledge of how the FPI/CYA work, this simply didn't make sense. So I plugged in a logic analyser and timed the Wolfenstein 3D code (NOP every 8 PEI's). I then modified the code to change it to a NOP every 13 PEI's, and finally to remove the NOPs altogether.
These are the results:
NOP every 8 PEI's - 40 PEI's takes 0.179005ms (4....