Gnus gets terribly slow when exiting high-traffic groups

[Jason L Tibbitts III <tibbs@math.uh.edu>]

I have a few some mail folders which have absurdly high traffic
(linux-kernel, a folder with logwatch output from every host I manage,
etc.).  Exiting those groups has started to take so long that I don't
even bother trying to read mail there.  We're talking over an hour on a
good CPU (i5-4690).

I had this problem a while back but it got better after I updated gnus.
Now I'm running the absolute latest version pulled from git a few
minutes ago.  This happens even if I enter a group that has had loads of
traffic but currently has no articles in it.  (It got big, I gave up and
unsubscribed, all of the articles ended up getting nuked by
total-expire, and so there's nothing in there now but the folder still
exists.  Entering it immediately goes back to the summary buffer with
"Expiring articles...")

debug-on-quit tells me it's sitting around in
gnus-summary-expire-articles().  It's not doing anything with the IMAP
server and the CPU is pegged.  Any idea how I can debug this, or get
things back to a usable state?  (I guess I can always rename the
folder.)  I once knew how to debug elisp but that's long been forgotten.

 - J<