Part of me wonders if we’re starting to see a limitation of ActivityPub? I might be wrong but I don’t see any sort of batching in the protocol?

Agreed. There is also a lot of protocol boilerplate overhead, digital signing, to send a single Like of a comment one to one server. Thee implementation also has no way for the server operator to know that another server is backing up or failing, and when the server is stopped - the queue gets lost.