The Z80 takes 3-6 clock cycles to execute a single instruction. The 6502 executed one instruction per clock cycle. So a 3.5MHz Z80 gives comparable performance to a 1MHz 6502.
The Z80 had more registers than the 6502, which is nice, but the 6502 instruction set included immediate values, which is super helpful.
Overall you can't argue about CPU performance without writing non-trivial programs which do the same thing in both Z80 and 6502 assembly and benchmarking them. The architectures are simply too dissimilar.
Fortunately people did do that at the time so we can see the results.
And the Spectrum had thousands of sophisticated 3d games running in real time and the C64... Didn't. It had the games it's sprite bitting hardware could support. People didn't even try the sophisticated software that was coming out on the Speccy.
...uhm... what were those "sophisticated 3D games" on the ZX Spectrum?
There were a lot of "isometric top-down camera" games with a static non-scrolling background, but those were hardly taxing, as the background was "slowly" rendered upfront and didn't need to update each frame.
The Z80 had more registers than the 6502, which is nice, but the 6502 instruction set included immediate values, which is super helpful.
Overall you can't argue about CPU performance without writing non-trivial programs which do the same thing in both Z80 and 6502 assembly and benchmarking them. The architectures are simply too dissimilar.