I'm talking here about the Fediverse and my experience with it (including ActivityPub).
Due to the data model: A server needs to store / understand all the data owned by the server. This is due to
- the nature of ActivityPub objects being fetchable by their id (a HTTPS uri).
- people often expecting a HTML version if they click on the object
This means all outgoing stuff needs to be tracked and understood by the server! Due to the desire of coordinating replies / like counts / etc ... the server also needs to understand the incoming traffic.