I think, by default, the focus should be on speeding up Guix (and specifically `guix pull`). I suggest to wrap %default-guix-channel inside channel-with-substitutes-available, like this:
(define %default-channels (list (channel-with-substitutes-available %default-guix-channel "https://ci.guix.gnu.org"))) This way, beginners would enjoy a little speed-up, and sophisticated users could still use (list %default-guix-channel). What do you think?