Local delivery seems to limit IMAP performance

[Gary Mills]

We run cyrus-imapd-2.3.8 in a murder configuration with one front end
and one back end server.  One thing I've noticed is that local message
delivery has a pronounced effect on IMAP client performance.  We've
had to limit the number of lmtpd processes to 64 on both servers to
maintain an adequate level of IMAP performance.  At peak times, we
will have over 6000 IMAP sessions and over 500 SMTP sessions on the
front end.  The result is that local deliveries are often delayed
during peak times.

What it is about lmtpd that causes it to affect IMAP client
performance so profoundly?  Can anything be done about this?

When I have an IMAP session running with mutt, it displays one line
for each message, and displays the contents quite quickly as I select
each message.  However, I've noticed that when I get a new delivery
(for which I get a separate notification), there's a long delay when I
select the next message.  It must be rebuilding some indexes at that
time, and fetching the new copies.  Is that correct?  What could we do
to improve this procedure?

-- 
-Gary Mills-        -Unix Group-        -Computer and Network Services-