Exchange Throttling Policy
If you need a higher execution speed of the EWS for migration purposes, then you could create a new throttling policy in the following way, which you can assign to the mailboxes you want to process.
Exchange 2010
New-ThrottlingPolicy ExMixedFoldersPolicy
Set-ThrottlingPolicy ExMixedFoldersPolicy -EWSMaxConcurrency $null
Set-Mailbox [MAILBOXNAME] -ThrottlingPolicy ExMixedFoldersPolicy
Exchange 2013 and up
New-ThrottlingPolicy ExMixedFoldersPolicy
Set-ThrottlingPolicy ExMixedFoldersPolicy -EwsCutoffBalance Unlimited -EwsMaxBurst Unlimited -EwsRechargeRate Unlimited -EWSMaxConcurrency unlimited
Set-Mailbox [MAILBOXNAME] -ThrottlingPolicy ExMixedFoldersPolicy