I've only really skimmed the ActivityPub protocol, but from what I understand the work done by the whole system scales with:
O(toots * mean followers / mean followers per server)
and the core scaling assumption is that 'mean followers / mean followers per server' scales sub-linearly with 'toots' to stop the end-to-end work being N^2.
Folks who've looked into it, does that sound right?