[Dovecot] Thunderbird and Dovecot on FC5 taking a long time to retrieve message

I'm running FC5 (2.6.15-1.2054_FC5smp) and Dovecot dovecot-1.0-0.beta8.2.fc5 with Thunderbird 2.0.0.16.

I set up imap (dovecot) so I can access my mail locally or remotely. It seems to take a very long time (minutes sometimes) for an Email to be retrieved after I select it in the folder list and/or in the Email list locally or remotely. I've traced the imap process and found it doing a lot of reading (read() system calls) for a long time. I do have a high message count in some of these folders (i.e.: 20000 or so). Is there any way to improve performance (use more memory, cache indexes, tune I/O parameters, etc...) ?

Thanks.

        -Mark