Hi J Sommerville,
I can fully understand that you’re trying to upload .PST files to Exchange Online mailboxes and you have perfect network capacity and system, but the upload speed is slow. Based on our previous support experience, the upload speed could be affected by multiple reasons when using this tool. To narrow down the root cause, we may need to involve some Technet forum engineers into this issue, such as collecting and analyzing related logs from PST capture tool. Their engineers are more familiar with this tool, and will be more proper and professional to help you narrow down the root cause. To sort out this issue more effectively, you can also try to post this question at the following Exchange server forum:
social.technet.microsoft.com/.../home
Once you get any feedback, or they can help you determine the root cause, please feel free to let us know. If there is any point that related to Office 365, we are glad to provide further assistance.
I appreciate your understanding, efforts and working closely with Office 365.
Thanks,
Young Yang