Eh... Windows 8 is actually the most efficient OS Microsoft has developed yet. It's the best OS for mobile devices. It runs on low spec machines fantastically well and scales superbly with better hardware. With DX12 coming, with low latency, low level API, Windows will scale even better with low spec to high spec hardware. It has more efficient use of power, threading etc than previous OS. It's support for drivers and switchable graphics is superb.