Hi ,
Although, it works correctly and I can browse FTP now, I cannot copy or paste from and to FTP smoothly. It takes too time and it is very very slowly.
For slow performance issue, tracing and monitoring logs should be necessary if there is no specific event/error about the problem. 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
Best Regards,
Candy