The error that you get refers to : ‘StalledDueToTarget_DiskLatency’
This means that it has to do with the Exchange Online servers and not with the On-premises infrastructure, so there is nothing you can do locally.

In this case the only thing you can do is open a case with Microsoft and ask them what can be the cause of this error from the target side (Office 365).

It would be a good idea to open a case with them mentioning the error and ask them if they can perform a change that might improve the migration speed.