By default Exchange 2010 moves 5 mailboxes at the same time. But you can change that. Go to %exchangeinstalldir%\V14\BIN and open with ultraedit, notepad or something like that the following file : MSExchangeMailboxReplication.exe.config
Look for the following (control-f :-)) : MaxActiveMovesPerTargetMDB =
Change the default number (2 or 5, depending which Servicepack) to a higher or lower number.
Restarting or something like that is not necessary.