It also claims that “the Z80 CPU at 3.25 MHz was actually faster than the Apple II processor (1.08 MHz).” This is, pedantically, true, in that the oscillator is ringing faster. However, the 6502 completes more work per clock cycle than the Z80 does. A single-byte instruction (such as “increment the A register”) takes 2 clock cycles on 6502, 4 cycles on the Z80. More complicated instructions can take up to 7 cycles on the 6502 and 20+ on the Z80. The estimate I use is that a 6502 is approximately 3x faster clock-for-clock than a Z80.
Also, the Apple II CPU didn’t spend 70% of its time drawing the screen! It had separate video hardware that transparently interleaved with the unused back half of the CPU clock cycle. The ZX81 was tricked into “executing” sequential screen-memory addresses in a very tight loop by falsifying its data bus with a NOP instruction. Fancier computers would use dedicated DMA hardware. Not Sinclair!
Also, the Apple II CPU didn’t spend 70% of its time drawing the screen! It had separate video hardware that transparently interleaved with the unused back half of the CPU clock cycle. The ZX81 was tricked into “executing” sequential screen-memory addresses in a very tight loop by falsifying its data bus with a NOP instruction. Fancier computers would use dedicated DMA hardware. Not Sinclair!