I'm interested in learning for a practical reason, not just theoretical
Well I was not a design engineer, but I did read a lot about digital speed increases (and the challenges they brought) over the decades as chips ran faster and faster.
One aspect, I think, is just raw edge transition speed. Even if you are only toggling a I/O pin at 1 mhz it's edge rise and fall times are determined by the fundamental internal chip clock speed, so likely to cause more problems when using a 80hmz chip then a 16mhz chip, no?
Lefty