Disable EWS throttling in both Source and Target tenants for better performance using the below steps.

• Login to Microsoft 365 Admin portal https://admin.microsoft.com/Adminportal/Home
• Navigate to Help using ? icon in the top right corner and enter “EWS Throttling Policy Increase” in the search and enter.
  
  
• Click ‘Run Tests’
• If throttling is enabled, it will provide you with the option to change the throttling policy. Provide the duration and click “Update”.
  
  

Run the test again after 15 minutes to confirm if throttling is removed.