Microsoft give Xbox One Developers a Christmas present: Another CPU Core to play with
Both the PS4 and the XBox One have eight CPU cores and from launch have kept two cores in reserve for Operating System resources and multitasking. The SDK, which appears to be from November, shows that Microsoft have released one of these cores to developers for use in games.
With additional power comes additional trade-offs: custom voice commands and Kinect functions cannot be used when the SDK is used to access this core. Part of the core is still used for "Xbox, Record That" and similar functions, so developers cannot be guaranteed 100% of the core at all the times.
Eurogamer have a detailed initial look at the effects of this change from Microsoft, head on over there for a look at the latest in the long improvements Microsoft have made to match the PlayStation 4 performance.