https://www.techradar.com/news/faster-xbox-one-game-pass-downloads-are-here-in-beta

Microsoft promised them at E3 2018, and Microsoft has delivered: faster Xbox One game downloads through Game Pass are here. Microsoft has upgraded its game download service with the new Fast Start feature, powered by its machine learning (ML) server technology.

Of course, this feature is only available at the time of writing within the Xbox One Preview Beta, with some members in the lower-tier Delta ring (where changes come more slowly but are more stable) having access. The Xbox One Preview works similarly to the Windows Insider Preview on Windows 10, allowing players access to less stable, new features before everyone else.