https://bonfirenetworks.org/ is a great ActivityPub-based forum/microblogging/real blogging/sharing software. It has all the features we need: threading, no character limit, unlimited alts, permissioned topics. It's written in Elixir, which is a nice messaging-oriented functional programming language.

Unfortunately, pages on the demo instance currently take about 13 seconds to load. There will need to be like a 20x speedup before it will be pleasant to use.

Will they ever manage that?

Resolves yes when most pages (comments, notifications inbox, message inbox, topic pages) load in less than one second. Resolves no if an equivalent piece of software (that isn't descended from bonfire) hits this level of development before bonfire does (the competitor also has to be open source and federated)