I have just set up a new laptop - Dell XPS13 running Ubuntu 16.04.3 LTS. I've noticed very high CPU usage when watching web video. From a fresh boot, I start a single Firefox (or Chromium) window, navigate to e.g.
and the video starts playing, and I watch (via htop) the CPU usage for firefox jump to about 90% and stay there.
I've reproduced this on both Firefox and Chromium, and on youtube, BBC iplayer and vimeo.