HN has a lot of optimizations and limitations on the user experience.
For example, I don't see the scores of every other post on HN - and certainly don't see them updated in realtime. New comments and posts show up when I reload the page - not as I scroll. There's only one HN instance - without having 100 more instances trying to mirror the content (just one - firebase).
... and HN is an example of a single, centralized, unfederated server.
The default Lemmy deployment docs don't have a lot of breathing room.
It would be reasonable to break off the postgres server Lemmy uses to a separate server, but at the moment there is only an Ansible playbook or a docker compose file, both of which assume a single server.
There's only so much time in the day and it's much easier to size up than reimplement the install process.
https://news.ycombinator.com/item?id=29927630