slow expiry: only 600 articles in group but gnus thinks there are 5000

[Dale Hagglund <rdh@yottayotta.com>]

I use gnus and nnml at work to read mail, and over a few years my
primary unfiltered folder (mail.misc) accumulated about 1900 ticked
messages in it.  It finally became too excruciatingly slow to expire,
so I moved a bunch of the articles to archive groups.  

mail.misc was still very slow to expire, and gnus still thought there
were ~5000 articles in it.  So, I marked all the articles with # and
used `B m' to move them all to mail.misc just to make all the article
numbers sequential.  This worked, but expiry is still very slow, and
gnus still says there are ~5000 articles when I type `C-u SPC' to
enter the group.

I've checked the .overview files and the directory listing in the
~/News/mail.misc directory, and the number of articles is really
around 600.  Also, I ran strace while gnus was exiting this group, and
for almost all that time, no file operations were being performed.
Eventually, a bunch of stat() calls were made for articles that don't
exist in mail.misc as well as for the 600 articles that do, but this
step only takes a couple of seconds.

It looks to me like there's some sort of quadratic algorithm going on,
and it's being run for the 5000 articles that gnus thinks are there,
rather than the 600 that are actually there.

Any ideas of what to look for?  I'm running ognus 0.07 on gnu emacs
21.

Dale.