The Z80 takes 4 clock cycles for an instruction fetch (or simple instruction) and 3 clock cycles for a memory access (if no wait states are triggered), while th

The Z80 takes 4 clock cycles for an instruction fetch (or simple instruction) and 3 clock cycles for a memory access (if no wait states are triggered), while the 6502 does a memory access in each clock cycle and a simple instruction in 2 clock cycles (in the end it's always been about memory speed). The Z80 may have been more convenient to work with because of the bigger instruction set and register bank, but a 2 MHz 6502 should easily be able to keep up with a 4 MHz Z80. The ingenous thing about the 6502 was its simplicity. It had around 3500 transistors compared to 8500 transistors of the Z80, essentially "small and elegant" vs "big and brute force". Disclaimer: I grew up on the Z80, but credit where credit is due ;)