"HN's main software runs on a single core", wrote a moderator when I wrote to ask about why rapid (<2s?) subsequent actions were rejected by the server (ie. like a post and then favorite it).

This was November 2020 and he mentioned a big overhaul was in the works (of both HN and Arc, the programming language in which it is written).

We've seen new features since then, but the throttling is still there so it seems to still be the same situation regarding fundamental infrastructure.