According to AMD's internal testing, the A10-6800K APU delivers about 3.5 percent more performance in PCMark8 on Windows 8.1 than Windows 8 thanks to new optimizations.
So from where does that uplift come? Our work with Microsoft includes development on the essential operating system "plumbing" that enables Windows to directly leverage AMD technology in order to run more efficiently. The two companies also cooperate on the development and tuning of the latest AMD video drivers.
Of course AMD's fast CPU and GPU cores contribute to high performance, but having software that is optimized to take advantage of the AMD hardware architecture is a significant advantage. Tuning our device drivers to simultaneously suit AMD hardware, software applications, and Windows 8.1 makes systems more streamlined.
Source: AMD Blog