> I run a benchmark for AES encryption - a modern CPU will have circuitry designed explicitly for this task and it's asm instructions. An old CPU just supporting the base x86 instructions probably doesn't have a hardware solution. Is it unfair to compare them?

YES! Unless you're specifically searching for the fastest AES cpu.

If you want to compare general performance this benchmark is flawed. E.g.: I could have the fastest CPU in existence, but since it happens to be lacking hardware AES circuitry, your benchmark will always show another CPU as the 'fastest'.

It's not 'unfair' or whatever. It just makes it so that you need to think better about your benchmark, what you want to measure, and what you're actually measuring. Or you need to adjust your conclusion: "this is the fastest cpu" -> "this cpu performs best on this specific task"