Chrome/Firefox: YouTube doesn't run very efficiently in Chrome and Firefox. In fact, How-To Geek points out, it's a battery hog. You can get around this problem with the h264ify extensions.
How-To Geek explains the problem with Google's VP8 and VP9 codecs, built into Chrome and Firefox to play videos, as opposed to the H.264 playback primarily used by Safari, Internet Explorer, and Microsoft Edge: H.264 playback (or decoding) is hardware-accelerated and can take advantage of your graphics processor (GPU). That means your computer processor doesn't have to do more work and less battery power is wasted. But devices that support hardware-accelerated decoding of VP8 and VP9 aren't as common (if even available).
If you want YouTube videos to play more efficiently and not waste battery life, you could either switch to a different browser or use the h264ify extensions to force your browser to request H.264 videos from YouTube.
Besides saving your battery life, playback should be smoother, particularly on laptops with limited power.
h264ify [Chrome Web Store]
h264ify [Firefox Add-ons]