Matrix has absolutely bonkers resource requirements relative to the end user experience it delivers, and can be much more complex to deploy. Some of that is intrinsic to federation or to its protocol; some is related to Synapse specifically. I’m not sure whether competing server implementations will catch up before Synapse gets a rewrite in a more performant language, but I’m eager for one or the other to happen.

I admin a public Matrix server, so I’m not by any means against the product, but IMO there’s a reason it hasn’t meaningfully displaced XMPP yet.

At this point federation traffic seems to have quieted down. I think what we’re dealing with now is just the expected behavior of Lemmy/ActivityPub in general: there’s not really any such thing as backfill. Old posts will appear as they’re commented on or users follow links to them, and (some) comments will populate as posts are viewed.

This has been raised as an issue but it’s never really gone anywhere: https://github.com/LemmyNet/lemmy/issues/3448

I think you’re correct that this is federation delay. When I’ve deployed Lemmy before, it’s taken a while for federation to stabilize (symptoms are community subscriptions showing “pending” and the lack of comments you described). I think this is due to limits on the amount of data exchanged at once. I’ll keep an eye on it over the next couple of days.