I have a system that filters RSS feeds like this, out of 7000 articles it might pick 300 to show me, it takes just a few minutes to build and validate a model on a rather powerful PC so so far performance is not a problem.

If I were wanting to offer this kind of service to other people I know I could get an economy of scale just ingesting RSS feeds once, I think a lot of users might be happier sampling items from the firehose instead of curating a feed list, and I'd probably give up on the pure content-based filtering and include some collaborative filtering because people would perceive that it learns faster.

That is exactly what is happening here. The Mastodon server is handling the firehose, and creating sub-streams for each user based on followed accounts and tags.

Video streaming seems like a firehose.

Do you mean the battery could not handle the LLM?