performance on large inboxes

[Ciprian Marius Vizitiu]

from time to time we have users with a very large inbox, which means it
contains 20.000 messages or even more. My quite general question is: 
What is cyrus doing once a user logs in through imap or pop3? It seems, that
it is parsing the directory, which takes very long. But what does it have
the indices for?
Of course I know that cyrus stores flags and headers in it, but why does
cyrus parse the directory if all parts that are fetched through the session
are the Date, To, From and Subject-header? Shouldn't that be in the index so
cyrus doesn't have to touch the directory except the cyrus-files?
Is there an option so that cyrus splits up big inboxes into several folders
so they can be read faster?


Slow logins can be equally blamed on the IMAP client. Take Outlook for
example: it insists into creating one giant .pst file (BTW: 2GB limited!)
for the whole IMAP account. The very first login in a 18.000 messages
mailbox becomes painfully slow if the client machine has a "slow" hdd (e.g.
2 years old laptop).