one report the console may require games to be installed, with a fee required if you want to install them onto a second account. Either way, going by this, players can boot games as soon as they start installing them.
In terms of instantly switching between games and the system's other functions like live TV and movies, as detailed during Microsoft's presser, the page confirms the Xbox One lets you suspend games "right where you left off, so you can resume instantly." Think how the Vita, Wii U, or 3DS work when you exit and suspend a game to use the systems' other functions. Except not with those systems, but with a shiny new Xbox.
Update: Microsoft told our friends at Engadget that Xbox One 50GB Blu-ray discs automatically rip to the system's 500GB hard drive, although it's not clear if installation is mandatory or not. Also, Microsoft Support indicated via Twitter the report mentioned above about installing games to a second account was inaccurate.