performance on large inboxes

[Marten Lehmann]

Hello,

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?

Regards
Marten