I don't think anyone is worried about the decoding requirements, which will be higher than the previous generation but only a factor of 2 or so.

Obviously, they won't be able to roll out high resolution content to phones, TVs or set top boxes until devices support it with hardware, which will be a couple of years from now.

But, there's plenty of people in the world with decent desktop power and terrible bandwidth who can benefit immediately from squeezing better video through their connection at the cost of more CPU at both ends.

Generally you won't hear those people commenting on forums like this, but they show up in companies like Netflix's business plans. YouTube has produced maps showing which countries had greater usage due to VP9.

I think them accepting a 5x encoding slow-down means they are mostly concerned about decoding speed.