There's a public delivery mechanism where if many people are on the same server, and its a public post (eg Twitter style messages, etc) it can be delivered to a shared public inbox https://www.w3.org/TR/activitypub/#public-inbox-delivery

That way the receiving server can only get POST'ed to once and distribute to the appropriate followers.

Of course, for more limited distribution posts, you still have to distribute individually, but that's no different than how email works.