Exchange 2010 and above has a low throttling limit and you may see errors or performance issues related to this.
If using Office 365, you can contact Microsoft and request that the limit for EWSMaxConcurrency is raised to the highest level possible (or removed) for your domain, or setup more admin users to avoid reaching the max concurrency.
You can also disable throttling parameters for the admin account by running the following commands in the Exchange Management Shell, replacing ADMIN with your admin user:
These items should be blank:
CPAMAXCONCURRENCY (EXCHANGE 2010 SP1)
CPAPERCENTTIMEINCAS (EXCHANGE 2010 SP1)
CPAPERCENTTIMEINMAILBOXRPC (EXCHANGE 2010 SP1)
To check which throttling policy is applied to the Admin account, execute the following command:
Get-Mailbox "ADMIN" | fl -property ThrottlingPolicy
Item Export Delay
You could also try implementing an item export delay in CloudM Migrate. This is found under Common > Other Options > Item Export Delay. We recommend entering a value in milliseconds from between 1000 to 3000, it's trial and error though, so start at 1000 and increase to 2000 or 3000 if it persists in failing.