> Speedwise the 1MHz 6502 and 4MHz Z80 were on par.

This is a bit of an exaggeration, the 6502 was efficient but not that efficient. While generally understood that the Z80 took 2x-4x ticks to execute instructions as the 6502, in the real world its larger register set meant properly-written Z80 code could avoid expensive, slow round trips to memory.

Outside of artificial benchmarks real world performance shows that the 6502 is roughly 2x as efficient per clock cycle as the Z80[0], i.e. a 1 MHz 6502 is approximately equivalent to a 2 Mhz Z80.

This is reflected in the computers of the day, i.e. TRS-80s were not being blown out of the water by Commodore PETs.

[0] https://github.com/soegaard/minipascal/blob/master/minipasca...