Recently I was engage in a project of migrating Exchange 2003 environment to Exchange 2010. User was having around 500 mailboxes on various capacities. During the mailbox migration process we found out the process was extremely slow. 500 MB mailbox transfer took more than 20 minutes!
Doing some search around found out this is a due a recent change on the Exchange 2010 SP1. The value of “MaxActiveMovesPerTargetMDB” was change from 5 to 2. This means only 2 mailboxes can be migrated simultaneously 
Not a good idea and so far no idea why Microsoft did that. You can edit this value in “MSExchangeMailboxReplication.exe.config” to 5 from 2.
Open the above mention file from a notepad and locate the entry of MaxActiveMovesPerTargetMDB and change the value.
Once you do that you have to restart the Microsoft Exchange Mailbox Replication service in HUB CAS servers. Try doing the Mailbox moving and you’ll find the difference.
Note: This information specifically given “AS IS” there is no such documentation found in official in Microsoft web site.
