So, if an activity is supposed to go to M actors, it would normally take about M deliveries to get it to all those actors. But if there are on average N actors that share a server, then using a shared inbox will reduce the total number of deliveries to M/N.
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!
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