It explains why at the second paragraph.

> A top level takeaway from this data is that for things that rely on pure Go the M1 machine is, in general, a good bit faster than the Intel machine. For code that relies on highly optimized assembly, like a good chunk of the crypto/ code, the Intel machine wins (although I wouldn’t be surprised to see significant gains for the M1 as arm64 assembly implementations get more attention and are further polished).

Basically Go lacks lots of optimized assembly code for ARM64.