major slowdown after upgrading to 2.2.10 from 2.1.17

[Huaqing Zheng]

After upgrading to cyrus-imapd 2.2.10 yesterday on my pilot server, I've
noticed a major slowdown in how the server responds to the LIST command.
Previously, when running 2.1.17, the LIST "" * command usually returned
all the mailboxes I have access to within a few seconds.  After
upgrading to 2.2.10, it looks like it is now taking about 30 seconds to
return the mailbox list on a server with 12000 mailboxes, with about
1500 of them shared.

Has anyone else noticed the slow down?  Any suggestions?  I grokked
through the source code but didn't notice anything majorly different in
the mailboxes.c or mboxlist.c.  I didn't bother going through the
database backend since I am using the same database (skiplist) format as
my mboxlist backend.

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