One can use this to import really big ones, dozens of gigabytes in size, imports which will crash, hang, and otherwise cough on Outlook very easily. Runs directly to folders inside of mailboxes. The amazing Yvonne Wynkoop found the first really good set of instructions we have seen:

blog.natfan.io/importing-psts

Mysteries do abound about the Microsoft-provided command line tool AzCopy, not the least being the fact that there is a version 10 and a version 8.1. Items as of this writing:

• Version 8.1 is downloadable from Office 365, and works. Have no clue what 10 is for.
• Usage and download of it, is now through Office 365 Security & Compliance, Information Governance, Import.
• When you run it for an upload, add “/NC:2” to the end of the command line. This increases its speed and stability quite a lot, and prevents timeouts. Default is reportedly 24. Perhaps the developers are sitting on Google Fiber?
• If it times out, just restart it carefully, it will usually continue where it left off.

If the above works for you, use the same number when downloading PSTs from eDiscovery, via registry edit:

https://docs.microsoft.com/en-us/microsoft-365/compliance/increase-download-speeds-when-exporting-ediscovery-results?view=o365-worldwide

Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Exchange\Client\eDiscovery\ExportTool]
"DownloadConcurrency"="2"