1.) Open an Exchange Management Shell
2.) Run the following cmdlets:
New-ThrottlingPolicy MigPolicy
Set-ThrottlingPolicy MigPolicy -RCAMaxConcurrency Unlimited -EWSMaxConcurrency Unlimited -EWSMaxSubscriptions Unlimited -CPAMaxConcurrency Unlimited -EwsCutoffBalance Unlimited -EwsMaxBurst Unlimited -EwsRechargeRate Unlimited
Set-Mailbox MigAdmin -ThrottlingPolicy MigPolicy
Where 'MigAdmin' is the name of your migration admin account.
3.) Restart the Exchange RPC Client Access service to activate the the new throttling policy settings.