It's been quite a while since I read about the inefficiency. I think it had something to do with CPU load, and that it's unnecessarily "chatty" in some ways that causes servers to use unnecessarily large amounts of data. And the extensions had to do with different types of services, where the AP spec is best suited for one type of service (like maybe micro-blogs iirc), and others have to use the spec in weird ways or add things on top of it to implement other features that are important for those other types of services, like more forum-esque type things like Lemmy. Don't remember exactly what they were, but one thing I read last week was that guy who had to shut his AP project down because he used a method of fetching data, that Mastodon (or whichever service it was) uses but isn't part of the AP spec, and poorly documented, so he implemented it wrong which had horrendous consequences for him, but that's a different story.