ActivityPub is push so it's more an issue with having a lot of followers, but it's one of those things where systems need to be upgraded and scaling solutions tried. A few months ago people like Kathy Griffin and George Takei went over as part of a Twitter exodus and things seemed to hold up ok.

To me the fediverse is pretty all encompassing, not all using AP, a decent proving ground, just not quite with the youth and ease of development of Nostr which is arguably part of the same decentralized gestalt.