Even on a fast connection the most you will be able to upload running a single instance of the Exchange Migrator is around 500 MB – 1 GB per hour. This is primarily due to throttling imposed by Office 365. In order to get more upload speed you must dedicate multiple systems to the upload process and each should be configured with its own Office 365 Admin account, which will reduce the likelihood of running into throttling issues.
For exporting messages from Office 365 to PST files, the speed will typically be around 1 – 2 GB per hour. Again the main limitation is throttling in Office 365.
Updated: April 16, 2016