A neat feature of the modern web is that Adobe Flash Player is (finally) starting to be replaced by HTML5 apps, and that includes video playback such as YouTube.
During this change I have, among allot of other people on the Internet, noticed that our favorite browser is not yet optimized for this future proof technology. And enabling (now default) HTML5 playback can lead to audio/video stutter and high CPU-usage.
Simply go to Settings > General > Performance and bump up the Content process limit (Uncheck Use recommended performance settings).
It seems, at least for me, help with the CPU utilization when playing HD/4k video on YouTube.
Read more at the support article: