Quick Resume allows you to go between multiple games and load straight into their suspend state. The demo shows several Xbox One games including Forza Motorsport 7, State of Decay 2, Ori, Hellblade and more. The official wording is that this feature will work with at least three Xbox Series X games.
The loading times video also uses Xbox One games to demo, but shows hugely increased loading times (of up to 20-30 second improvements loading into a game). This is to be expected due to the brand new NVME SSD that the Xbox Series X uses. It’s unclear how this will diff with Xbox Series X games, but there should be a fairly decent improvement.