There are various benchmarks on YouTube showing the performance is reasonable. Native code is obviously faster but with rosetta it seems like the M1 is a mid tier Core i5 or Ryzen 3 series in performance. Not bad at all but not top tier.
IBM POWER9 supports some memory ordering instructions that (as I understand) would in theory be useful for x86 emulation, but a) I'm unsure if anyone actually uses them and b) They are removed from POWER10