In ActivityPub, each actor has an optional `sharedInbox` endpoint, that is used for this shared inbox delivery. If an activity is supposed to be delivered to 10 actors on the same receiving server, it only gets delivered once, and the receiving server routes it to those 10 actors directly. We expect that internal routing to be much faster!
Another issue with ActivityPub is that we have a special delivery address, `Public`, which means that the activity should be visible to _everyone_. One question that arises is, if it's visible to everyone, should it be *delivered* to "everyone"? And if so, how is "everyone" defined?
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