The University of Minnesota reports that a performance issue was
discovered in Outlook 2003 when several (or large) IMAP folders are
subscribed. The problem arises when Outlook checks for mail, scanning
not only the Inbox but also all the IMAP mail folders stored on the
server. Combined with a frequent mail check interval or a relatively
slow Internet connection, this unfavorable configuration can cause
slowness and timeouts in Outlook.

It recommends lengthening the time for automatic Send / Receive.

It recommends unsubscribing from folders and removing the check from
the checkbox "When displaying hierarchy in Outlook, show only
subscribed folders."

The issue of working offline may be due to folders being unchecked
under mail options Send/ Receive. Outlook creates an offline file, or
ost file, if the box is checked.