Moving mailboxes is an exchange task that will by default be limited to moving (multi-threaded) 4 mailboxes at the same time. It is however possible to increase that number by adding the parameter MaxThreads to the Exchange Management Shell cmdlet Move-Mailbox. The MaxThread parameter will define how many threads can be used when moving mailboxes, any number between 0 and 30 can be set. The following single Exchange Management Shell line will retrieve all mailboxes located on a Exchange 2007 Mailbox Server called MailServer01, move them to a targetdatabase called "Vips" on another Exchange 2007 Mailbox Server called MailServer02, and 8 mailboxes can be moved at the same time.
Get-Mailbox -Server MailServer01 | Move-Mailbox -TargetDatabase MailServer02\Vips\Vips -MaxThreads 8.
Be aware though, that you need to test your exchange environment, to make sure you define the acceptable value for this parameter.