I was wondering the same thing! Especially considering modern messengers all talk HTTP any way. I think it should be possible. For example, federation in Plemo

I was wondering the same thing! Especially considering modern messengers all talk HTTP any way. I think it should be possible. For example, federation in Plemora seems lightning fast on a local instance. (Mastodon is a bit heavier and slower it seems. Then again, I’m currently fighting interop issues with Plemora instances.) So it seems it should be possible to build a decently responsive messenger with ActivityPub.