That's because of the kludge that Veritas came up with to do brick level (mailbox) backups
Backup exec has to open an Outlook session, then attach to each mailbox in turn and READ each item using standard MAPI calls. This is EXTREMELY inefficient, especially over a network. We've seen reports of 10-12 MB/min on decent hardware for brick level backups
If you have windows 2003, and enough disk space, you can use the Recovery Storage Group to restore the whole data store and export the desired mailbox(es) to a pst and import into the production system
on 2000 or 5.5, about all you can really do is have a standby server to do the restore to, then again export to a PST
Read through hhttp://mail.tekscan.com/nomailboxes.htm for more detail.