Hi @angus, I can't be certain about what is actually happening, which needs an admin and knowledgeable person with the ActivityPub plugin, but I observe that this forum becomes slow or even unreachable at times and it looks to be related somehow to posts that are being federated.
Notified about that in this post, saw it happening yesterday after @silverpill tooted this reply, and I got "503 Gateway timeout" from NGINX which I think may relate to this post.