Are we really ready to declare that Lars Magne Ingebrigtsen is a substandard Emacs Lisp coder?
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.
Lars is amazing by the way, his relentless Emacs work and prolific bug killing in the last few years should be praised to high heavens.
This means Emacs always has an up to date view of my email within a ~30 second window of the email being received and all I'm asking it to do is query mu which takes ms.