faster is a modern PC than a 4MHz Z80 from the mid-1970s? What proportion of this speedup is due to clock speed, and how much is due to other factors?...

How many times faster is a modern PC than a 4MHz Z80 from the mid-1970s?

What proportion of this speedup is due to clock speed, and how much is due to other factors? Comparing the clock speeds should be straightforward; superscalar execution and increased clock-cycle efficiency may also contribute