Exchange 2010, by default, allows you to do 2 concurrent mailbox moves.  This is a rather conservative approach, in this age of 10 Gb Ethernet and super-fast SAN storage.  Fortunately Microsoft has provided a mechanism whereby we can increase the amount of concurrent moves allowed.

This can be re configured by editing the MSExchangeMailboReplication.exe.config file, which resides under the C:\Program Files\Microsoft\Exchange Server\V14\Bin\ folder.

Attribute values:

MaxActiveMovesPerSourceMDB: This property indicates the number of mailboxes that can be moved on the source mailbox database at one time. The default value is 5 concurrent moves.

MaxActiveMovesPerTargetMDB: This property indicates the number of mailbox moves that can be moved on the target mailbox database at one time. The default value is 5 concurrent moves.

MaxActiveMovesPerSourceServer : This property specifies the total number of tasks MRS can perform that include the server as a data source. You can specify a value from 0 through 1000. The default value is 50 concurrent moves.

MaxActiveMovesPerTargetServer: This property specifies the total number of tasks MRS can perform that involve the server as a data target. You can specify a value from 0 through 1000. The default value is 5 concurrent moves.

MaxTotalMovesPerMRS: This property specifies the total number of tasks that a single instance of MRS can perform at a time. You can specify a value from 0 through 1000. The default value is 100 concurrent moves.

Don’t forget to restart the Microsoft Exchange Mailbox Replication service and do this configuration change on all your CAS servers

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