Raspberry Pi interview: Eben Upton reveals all (Linux User)

The same description can be given to many thousand-dollar laptops I know regarding the CPU: Video decoding is wildly inefficient on the CPU versus the GPU/DSP, that's the nature of the beast. And, in fact, what GPUs/DSPs are created to assist with anyways. I've played 1080p video on my laptop with the CPU sitting at 800Mhz and all but one core switched off specifically because the GPU did it, if I used a software-decoder the system barely survives at 2*2.0Ghz chugging away.

CPUs are general purpose, but that doesn't mean they're maximally efficient for all workloads, especially insanely parallel ones like video decode/3D rendering.