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