Hey there micah,
assuming you have your Dovecot set up using virtual users, I would advise you to use imapsync for that. It does incremental and recursive IMAP transfers from one mailbox to another (on different servers too). As it doesn't make a file copy, but let the two dovecot servers talk directly to each other via IMAP, all your mails with all flags, dates etc. will be preserved on the destination system exactly like on your source system. In your case also the indexes will be written on the fast system automatically by Dovecot 2.2.5 (if configured to do so), so this might do exactly what you are looking for.
I have myself successfully transferred over 35000 accounts with imapsync (with very few problems, almost all of them because of corrupt mailboxes or special character encoding problems occured through former use of weird obsolete mail clients) and can highly recommend it.
Best regards and good luck, Megodin
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