I switched to nognus for better speed, and recently made the jump to mu4e with offlineimap. And I'm already in love, so fast and non-blocking when refreshing emails! Of course, I could have had offlineimap with gnus, but it was a tedious setup (when I checked) and mu4e was quite a breeze. Local indexed search is so good I'm don't care that much now about filing emails anymore.
Unless I am very much mistaken, cmuscheme.el (a.k.a. Inferior Scheme mode) and all the other modes derived from Comint do not ever lock up Emacs.