By default, Exchange 2010 Client Access Servers throttle how many mailbox moves can concurrently migrate to Exchange 2010.  This throttling is controlled by the Exchange 2010 CAS Mailbox Replication Service (MRS) and limits both the target and source Exchange servers performing the moves.  In my example, I migrated 45,000 mailboxes from Exchange 2003 and 2007 to Exchange 2010 SP1.   When I first began the migration we moved only 35 per hour @500 MB mailbox averages.  For an organization this large, the migration would have taken months.

 

In order to override the default migraiton throughput, you can follow this article:

http://technet.microsoft.com/en-us/library/ff963524.aspx.

 

“The Mailbox Replication Service (MRS), which resides on all Microsoft Exchange Server 2010 Client Access servers, is the service responsible for moving mailboxes, importing and exporting .pst files, and restoring disabled and soft-deleted mailboxes.”

If you go to your CAS server and navigate to the c:\Program Files\Microsoft\Exchange Server\V14\Bin\MSExchangeMailboxReplication.exe.config file using a text editor such as Notepad, you will see the following settings at the bottom of the config file.

 By default, the CAS servers are set to:

MaxActiveMovesPerSourceMDB = “5”    

MaxActiveMovesPerTargetMDB = “2”    

MaxActiveMovesPerSourceServer = “50”    

MaxActiveMovesPerTargetServer = “5”    

MaxTotalMovesPerMRS = “100”

 

Essentially, these settings only allow 5 concurrent moves from the source database, 2 concurrent moves for the target database, 50 per sources server, and 5 per target server, and 100 moves per CAS server.  If you are running high-end disk or have plenty of IOPS to spare, you can change the settings to:

 

MaxActiveMovesPerSourceMDB = “15”    

MaxActiveMovesPerTargetMDB = “15”    

MaxActiveMovesPerSourceServer = “50”    

MaxActiveMovesPerTargetServer = “40”    

MaxTotalMovesPerMRS = “250”

Once changed you must restart the MRS services on the CAS server.  After changing these settings, we began to see mailboxes moves jump from 35\hour to 200-250\hour.   I do want to note that the migration process is very disk I\O intensive so you should adjust these settings gradually and account for any specific hardware limitations in your environment.

 

https://edmckinzie.wordpress.com/2011/12/12/how-to-speed-up-a-large-exchange-2010-migration/