[jonsen]

1 MHz clock on the 6800 family of processors, including the 6502, is equivalent to 2 MHz clock on the Z80. The 6800 architecture used a so called two-phase clock. Each phase, positive and negative, of the clock cycle was used to do work.

[timbit42]

I would suggest it is between 2 and 3 times, but it depends on what kind of code you are running.