Offline database maintenance can take a significant amount of time typically one hour for every 5 gigabytes of information.  Not to mention your database has to be offline this entire time for the process to complete.  To circumvent this issue and administrator can create a new information store on the same server or another server and simply move all the mailboxes to this store.  Because it is a newly created store it is in optimal condition.  This reduces downtime as only the mailbox currently being moved cannot be accessed.  Once this is complete simply delete the old database file and you have optimized your database.