To put it into perspective, 64KB of DRAM contains 524288 transistors, as 1 bit of DRAM needs 1 transistor. The 6502 is mid-1970s technology while the 64KB DRAM is early-1980s technology.
Despite having so few transistors, the 6502 is generally considered at least twice as fast as the Z80 for the same clock speed [https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/the-cpc-revision-zero-article/msg243843/ Source]. Three Four reasons explain this:
* The 6502 has an 8-bit ALU, while the Z80 uses a 4-bit ALU.
* The 6502 features a built-in clock doubler, allowing it to perform 1 internal operation and 1 memory access per cycle.
* On average, a 2MHz 6502 can make 2 memory accesses per microsecond, while a 4MHz Z80 can only make 1.
* The Z80 has more registers but has to do pretty much everything with them whilst the 6502 can directly use the first 256 bytes of memory for those jobs.
This shows the design differences between the 6502 and Z80. The 6502 focused on efficiency, using its limited transistor budget for the ALU and fast memory access. In contrast, the Z80 neglected these and used more transistors for extra registers and complex instructions. However, these Z80's features, like the alternate register set, register pairs and 16-bit stack, make it easier to develop complex software.