https://www.theverge.com/2018/7/5/17536732/microsoft-xbox-one-july-update-faststart-feature

Microsoft is releasing its Xbox One July dashboard update to all users today, and the main new feature allows Xbox owners to start games that are downloading quicker. FastStart, originally unveiled at E3, prioritizes the files a game really needs to start while it continues to download the rest of the title in the background. It requires a download speed of at least 20Mbps, and will be available for a “select number” of popular games that are available on Xbox Game Pass.

“You can generally expect to jump into your game twice as fast as you did previously,” explains a Microsoft FAQ on the new feature. Microsoft has not yet provided a full list of games compatible with FastStart, but the company says it plans to make it available to more games over time once titles are updated. FastStart is separate, and supplants, the existing “ready to start” feature that allows some Xbox One games to start before they’re fully downloaded. Ready to start typically let gamers play an initial level before the full game was available.