In the end, you could do memory load/stores with around 1 byte per microsecond on most home computers, no matter if 6502 or Z80. The Z80 did less work per clock, but was usually clocked 2..4x higher than 6502 based systems, which made up for the higher instruction cycle counts (the 2 MHz 6502 systems might have a slight edge against a 4 MHz Z80, but it wasn't night and day, and the C64 got away with its 1 MHz clock because the CPU was more or less just a controller for the various custom chips).
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In the end, you could do memory load/stores with around 1 byte per microsecond on most home computers, no matter if 6502 or Z80. The Z80 did less work per clock, but was usually clocked 2..4x higher than 6502 based systems, which made up for the higher instruction cycle counts (the 2 MHz 6502 systems might have a slight edge against a 4 MHz Z80, but it wasn't night and day, and the C64 got away with its 1 MHz clock because the CPU was more or less just a controller for the various custom chips).