Many Office 365 migrations use Exchange Web Services (EWS) to move data into the service. Often, these migrations run into throttling limits impiosed by Office 365 to conserve resources. Microsoft would lift the restrictions if you asked them by creating a support ticket. Now the Support Assistant offers an automated way to lift throttling for up to 90 days.
https://office365itpros.com/2020/06/09/microsoft-lifts-ews-throttling-for-migrations/
