The error that you get refers to : ‘StalledDueToTarget_DiskLatency’
To be straight to the point this is an issue where you can do nothing about. Link
When you get the message 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. When you have done this 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 (StalledDuetoTarget_DiskLatency) and ask them if they can perform a change that might improve the migration speed.
Click here to read other posts for more Exchange related posts.