Step by Step Directions for Techs
Migrating between Exchange 2007 and 2013 or Exchange 2010 and 2013? Mailbox migration speeds can be unbearably slow.
Exchange 2013 (as well as older versions, including 2010) changed the number of concurrent mailbox moves to 2… let’s change that to 5. Beware of setting it too high as it could have a serious impact on the Exchange infrastructure.
This change has to be done on all the front end servers. You should see that it will now migrate 5 mailboxes simultaneously. You can also speed things up by migrating to multiple data stores simultaneously. Some caution is needed when trying to max your migration speed as you can pretty easily overrun the storage (especially if it is local).