It seems your issue is that slow copy performance between two servers. For slow copy performance, we need to analyze performance log and network traffic to find the cause. However, analysis of log is beyond our forum support level and due to forum security policy, we have no such channel to collect user log information. So, we recommend you open a case with MS Professional tech support service, they will help you open a phone or email case to Microsoft, so that you would get a technical support on a one-to-one basis while ensuring private information.
Here is the link:
https://support.microsoft.com/en-us/gp/customer-service-phone-numbers