It uses postgres for the DB - I think that and redis are designed to operate at very large scales, so it wouldn’t be them.

My guess would be that it’s something in the interpreted nature of Python - this seems to be why a familiar dismissal of PieFed is a concern about how it will scale.

That said, this site shows that Python is the most popular language for Fediverse apps (just), the likes of Mastodon are written in another interpreted language (Ruby), and I think there are more big websites running Python (with Django or Flask) than people realise. So I don’t know, really, I’m just following other people’s lead on this. I don’t imagine that any problems would be insurmountable though: an admin could restrict the amount of signups, or if new users mean a few more donations, they could just throw money at the problem (more cycles for one server, or splitting up tasks across multiple servers).