Does the polling need to be fast? I think back to mailing lists and the huge delays involved in those conversations. Yet they were/are often very productive. Somewhere between Twitter/X speed and mailing list speed might be acceptable.

Maybe this would be better with a LiveJournal style interface. Medium length posts with threaded comments/replies are an underrated format.

That should scale pretty well. The HTTP fetch of posts/index.json could use conditional get requests to avoid downloading the body when there are no changes. Static files are dirt cheap to serve.

Maybe that's a feature rather than a bug