MITS 2SIO serial chip?

If you run a 20 MHz Z80 against a 20 MHz 6502, you'll find the 6502 performs WAY (3x-5x) faster than the Z80. It's difficult to base a comparison on clock rate alone. All 6502 memory cycles take 1 clock tick. Z80 memory cycles, aside from M1, take 3 clock ticks, with no wait states in use. If you run the 6502 at a rate that fully utilizes the memory bandwidth at a rate such that the Z80 can perform an M1 in the same amount of time, without wait states, the 6502 will always be faster, because it's running faster. The Z80 uses 1-1/2 clock ticks to execute its instruction fetch. If that's to be memory access window for each processor, and you run them both from static memory, and you allow minimal recovery time, e.g. use 10-15 ns memory, (just for the comparison) then you can use a 20 MHz Z80 and a 20 (actually 14) MHz 6502, and clock the 6502 with a 25ns low, 75ns high clock, and drive the Z80 with a square 20 MHz. That will be a 10 MHz clock for the 6502 and a 20 MHz clock for the Z80. I'd submit that the 6502 will still run rings around the Z80, since it is still going to be cycling memory at an average of 200 ns per cycle, while the 6502 does it a 100 ns rate.