Sure, specialised hardware tends to be more efficient than general purpose hardware but it's not so much "special-purpose hardware is magic" as it is "the general purpose hardware doesn't take advantage of certain properties". As far as I understand the GPU's advantage over CPU (again, graphics is not my area of expertise) it's largely that graphics processes are massively parallel and the GPU is specifically set up to handle massively parallel things. Like I said, I know nearly nothing about video encoding/decoding so I was hoping someone who did could explain things for me. My only guess for how specialised hardware could dramatically save power for video decoding is that it might be comprised of massively parallelisable simple operations.