Gnus has been around since the mid 90s (and predates the addition of asynchronous url-retrieve api to emacs). It is unfair to compare it with packages that written in the last couple of years.

I don't use Gnus so I have never bothered looking at its internals, but if it is locking your emacs for twenty seconds checking email, it is probably not doing much asynchronously. It is notoriously hard to convert existing synchronous code to asynchronous without using threads. Promises/futures/continuations/whatever-you-want-to-call-them are a pretty all in affair. Not having the time investment that would take to convert a 100k line codebase to async does not make Lars Magne Ingebrigtsen a substandard Emacs Lisp coder.