One of the exciting things announced a year ago was the introduction of the new Microsoft SharePoint Online Migration API. Instead of using the front-end to migrate content, the Microsoft SharePoint Online Migration API uses a dedicated network path through Azure to dramatically speed up copying of content to Exchange Online, OneDrive, or SharePoint.
However, there is a key rule to understand before you plan your migration – only items are copied through the Migration API. The structural elements are still copied through the slower front door.
Why is this important? Think of the amount of content vs. structure of a corporate communications intranet. There may actually be that much content (we find many cases where the intranet is less than a couple gigs) but the structure can be quite complex with lots of master pages, page templates, lists, sites, content types, etc. When you copy all this structure, its still going to be quite slow to copy because these structural elements are recreated through the CSOM API and throttled to ensure performance of the tenant.