Sublemmy? Slemmy?

Yepp, it works surprisingly well. I assume one of the similar communities will eventually “win” on one of the instances, like with similar subreddits over time. Also some instances will go full specific, like nature or movies or gaming etc. See the growth of lemmynsfw already, lol.

I’m really liking it a lot. I wasn’t too amused by Mastodon either, but as you say: for link aggregation, for specific communities, for discussing topics (and not being about people, but about topics) this is a perfect match.

I’m just speculating of course, too, but could be some kind of sharding e.g. in the DB level. I can imagine the little subreddits draw little traffic hence fewer shards are allocated to them (like how S3 works).