It can't scale without spinning up new (heavy) instance. Its database is hosted in the same <<thing>> as the web server ( https://join-lemmy.org/docs/en/administration/install_docker... ). Note that it's a single container that exposes port 80 and 443 - the database is hosted inside of that container.
That makes scaling the main instance impractical without redesigning how its built, deployed, secured, and connects to its backend database.
Note also that adding other sites increases this load without adding users as polling for content constantly is likely more expensive than having a user connect directly (and use it for a limited time).
Why does it need to scale? Does my laptop need to scale when I open a 100 megabyte file? No, it just does it in a few milliseconds then goes back idle. What I mean is, 1500 users sounds like a load you handle with a Raspberry Pi in a forgotten corner. I just don't understand what is happening to overload even the weakest of cloud instances?
That makes scaling the main instance impractical without redesigning how its built, deployed, secured, and connects to its backend database.
Note also that adding other sites increases this load without adding users as polling for content constantly is likely more expensive than having a user connect directly (and use it for a limited time).