Debugging extremely long IMAP folder exit times

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

Occasionally a folder on an IMAP server will get into a state where it
takes ages for gnus to leave it.  I have one folder which before today
was quick to exit, but today suddenly takes over two minutes.  Gnus
shows "Expiring articles..." in the minibuffer.

If I C-u RET to enter the group, gnus thinks there are over 364000
messages in the group.  There are actually only about 9000 messages
there, according to another IMAP client.  My guess is that gnus is just
iterating over 364k entries to see what it should expire.

Note that when I run gnus-group-expire-articles on the group from the
summary buffer, it doesn't take very long at all.

Here is what's in newsrc.eld for the group.  I was worried there would
be a huge set of ranges in there but the data is quite simple.

("fail2ban" 3 ((1 . 364570)) ((unexist) (seen (364431 . 364570))) nil
((modseq) (uidvalidity . "1535374911") (active 1 . 364570)
(permanent-flags %Answered %Flagged %Draft %Deleted %Seen %*)
total-expire (expiry-wait . 7)))

Any hints on understanding what gnus is doing here, or suggestions on
how to speed it up?  I used to know quite a bit about Emacs and Gnus
internals but it has been many years now since I tried to do anything
interesting.

I am running emacs 26.2; gnus-version says "Gnus v5.13".

Thanks,

 - J<