Game developers will be able to use that GPU time in the future, though.    



From ArsTechnica:

The Xbox One's ability to run up to four apps in the background (or on the side via Snap mode) during gameplay and to switch from a game to those apps almost instantaneously obviously comes at some cost to the system's maximum theoretical gaming performance. Now, thanks to an interview with Xbox technical fellow Andrew Goossen over at Digital Foundry we have some idea of the scale of that performance cost.

"Xbox One has a conservative 10 percent time-sliced reservation on the GPU for system processing," Goossen told the site. "This is used both for the GPGPU processing for Kinect and for the rendering of concurrent system content such as snap mode."

  System processing takes up 10 percent of Xbox One GPU time