If every solo user spun up an instance just for themselves there wouldn’t be a benefit over all those users just signing up directly to an existing instance.

Not quite true… There’s a ton of sql queries that go on to show you the correct content for your user. Showing the correct subscriptions, applying the correct “bans” etc… Further sessions management of the user logging in itself… Direct messages, inbox management… etc…

Just the raw content being sent to sync is just minimal text data that’s effectively broadcasted. The stress of that is virtually nothing in comparison.

The break even is that a ton of data might sync that you never look at… but it’s a broadcast of sorts. So that’s a bit of a non issue.