VP9 on Youtube has been awful as far as cpu use goes (and by extension, outright murderous on laptop batteries), as there are very few hardware decoders available. I'm bordering on calling Google ***holes for a rather strongarm method instead of including an option for h.264 (in their html5 player), however, VP9 as an open source and royalty free competitor to HEVC is a major plus.
That said, my i5-460M in my laptop averages roughly 40-50% cpu usage with full screen 1080P VP9. While the already weak (aged) battery life is completely abyssmal in Youtube now, I'm quite glad to see that cpu aged pretty well.