The part that worries me about scalability in the long term is the push nature of ActivityPub. My server is already getting several POST requests to /inbox per second already, which makes me wonder how that's gonna work if big instances have to push content updates to thousands of lemmy instances where most of the data probably isn't even seen. I was surprised it was a push system and not a pull system, as pull is much easier to scale and cache at the CDN level, and can be fetched on demand for people that only checks lemmy once in a while.
I think there's a benefit to the push model, as the instances can prioritize who to push to first if there's scaling issues, instead of having to throttle GETs, effectively the end result is anyway the same that nothing ends up to other instances in real time (which is fine). I don't know how lemmy works exactly, but could the push model just be a detail of activitypub https://flak.tedunangst.com/post/what-happens-when-you-honk ?