By default Exchange 2010 just move 5 mailboxes simultaneously. To increase or decrees this it you have to change a configuration file for Mailbox Replication Service.

Open the file C:Program FilesMicrosoftExchange ServerV14BinMSExchangeMailboxReplication.exe.config;

Increase the values to a number of simultaneous active moves that you want.

These values are:

MaxActiveMovesPerSourceMDB, MaxActiveMovesPerTargetMDB, MaxActiveMovesPerSourceServer, MaxActiveMovesPerTargetServer, MaxTotalMovesPerMRS

<!– Mailbox Replication Service configuration

Setting Name – Default, MinValue, MaxValue

MaxRetries – 60, 0, 1000
MaxCleanupRetries – 5, 0, 100
RetryDelay – 00:00:30, 00:00:10, 00:30:00
MaxMoveHistoryLength – 2, 0, 100
MaxActiveMovesPerSourceMDB – 5, 0, 100
MaxActiveMovesPerTargetMDB – 2, 0, 100
MaxActiveMovesPerSourceServer – 50, 0, 1000
MaxActiveMovesPerTargetServer – 5, 0, 1000
MaxTotalMovesPerMRS – 100, 0, 1024
FullScanMoveJobsPollingPeriod – 00:10:00, 00:03:00, 1.00:00:00
MinimumTimeBeforePickingJobsFromSameDatabase – 00:00:04, 00:00:00, 01:00:00
ServerCountsNotOlderThan – 00:10:00, 00:00:00, 01:00:00
MRSAbandonedMoveJobDetectionTime – 01:00:00, 01:00:00, 12:00:00
BackoffIntervalForProxyConnectionLimitReached – 00:30:00, 00:00:30, 1.00:00:00
DataGuaranteeCheckPeriod – 00:00:10, 00:00:01, 02:00:00
DataGuaranteeTimeout = 00:30:00, 00:00:00, 12:00:00
DataGuaranteeLogRollDelay = 00:01:00, 00:00:00, 12:00:00
EnableDataGuaranteeCheck = false, false, true
DisableMrsProxyCompression = false, false, true
DisableMrsProxyBuffering = false, false, true
MinBatchSize = 100, 2, 1000
MinBatchSizeKB = 256, 16, 16384
–>

Save and close the file;
Restart the Microsoft Exchange Mailbox Replication service.