On Twitter, the reviewer blamed it on Ryzen's L3 cache being a victim cache [1] (which I don't really understand; maybe it's related to having ephemeral data structures displace long-lived ones?). I think it's also been stated elsewhere that the Intel chips dominate during linking, which is a large chunk of the overall build time for Chromium.

[1] https://twitter.com/IanCutress/status/868799386079420416