How can we speed up message searching and message moving given the following:

Using:
Maildir on Linux ext3 (with Indexing enabled)
Courier IMAP; Thunderbird version 2.0.0.12 (20080213);
Have resynchronised all mail folders after deleting the .msf files.
Have compacted all folders.

Some of the folders contain 1000's of messages but surely this should not be a problem?!?!?

Symptoms:
Opening folders is slow - e.g. 15-30 secs.
Moving messages is slow - e.g. 6-10 secs.
Searching folders is very slow - e.g., 40 sec - 1 minute (though sometimes is goes on ad nauseum).

Is this a problem with maildir or imap or t.bird???

ANY help would be greatly appreciated.

Thank you.
k.