ActivityPub seems to require a lot of hardware resources in order to run properly, which is unfortunate. It’s not something I would ever want to run myself, especially to the public.
Versus something like Pleroma; which I've used since it's inception, being incredibly janky and lightweight, prone to breaking, but later versions have mostly ironed out most of those catastrophic bugs. It has it's own challenges as well, but it does historically scale better, is more flexible, and less intensive per instance
One demands a lot of money and time, where the other demands a lot of time and not so much on the money side. I'm not going to spend the time to give you a history of pleroma/mastodon instances, as it's a controversial history at best and there's a lot of people who know little, yet who will believe themselves an oracle. (ofc that could also be me so take it with a grain of salt and all) Though if you are willing to read through a bunch of highly opinionated accounts, and you pay close attention to what actually happened, the answer is pretty clear.
ActivityPub is intensive, but not the main culprit.
Correction: "The Mastodon software requires a lot of hardware resources in order to run properly"
Alternatives like GotoSocial, Akkoma, even Honk are much less resource intensive.
I don’t mean to be glib, just wondering if things can be “done better”
There's some additional overhead (doing HTTPS calls for verifying signatures, for instance) but that information can be cached pretty effectively.
Pushing contents is no more than posting HTTPS calls to every server in your follow lists, and possibly exposing said content in a GET API for pullers, though that's entirely optional.
Mastodon is heavy because of the way the backend is written (I blame Ruby on Rails for tha one), but there are fully featured ActivityPub servers out there that are orders of magnitude more efficient. Mastodon devs prefer the ease of development over performance but that's a choice, not an inherent problem of ActivityPub.