h264ify is a Chrome extension that's been around for a while. What it does is force YouTube to play videos using the h264 codec, rather than VP9, as the former has wider hardware acceleration support. Unfortunately, if you're still on Windows 7 or 8.1 and use Chrome, it doesn't matter if you bought your GPU yesterday — VP9 won't be hardware-accelerated in Chrome.
Basically, hardware-accelerated VP9 is disabled in Chrome if you're running Windows 8.1 or older, as it "isn't supported well". You can check yourself to see if it's on or not by opening
chrome://gpu in the browser and looking for the following warning:
VPx decoding isn't supported well before Windows 10 creators update.: 616318, 667532 Applied Workarounds: disable_accelerated_vpx_decode
If it's there, it means Chrome will fall back to software VP9 decoding on YouTube, which puts higher demands on your processor. Not a big deal on modern hardware, but on older PCs (such as my ageing i5-750), this can represent a good 15-20 per cent of CPU time.
The good news is by installing h264ify, you can force YouTube to use h264, which is accelerated on everything, short of actual potatoes. The only downside is that YouTube doesn't support resolutions greater than 1080p when using the older codec, but it's easy enough to switch the extension off for 4K content.
h264ify [Chrome Web Store]