Microsoft has revealed that 10% of the GPU's power on the Xbox One is reserved for the Kinect and the system's snap mode. Microsoft's Andrew Goossen informed Digital Foundry of this system reservation.
"Xbox One has a conservative 10 per cent time-sliced reservation on the GPU for system processing. This is used both for the GPGPU processing for Kinect and for the rendering of concurrent system content such as snap mode."

"The current reservation provides strong isolation between the title and the system and simplifies game development - strong isolation means that the system workloads, which are variable, won't perturb the performance of the game rendering. In the future, we plan to open up more options to developers to access this GPU reservation time while maintaining full system functionality."

More from Goossen, and a fairly detailed investigation as to what this all means for developers and the true power of the Xbox One, can be found over at Digital Foundry.