In a lot of distros, vdpau which accelerates video through the GPUis often not installed even in recent distros. This will result in Firefox with Youtube dropping frames when you check the "Stats for Nerds" and to the eye as jerky or slower video playback due to the dropped frames.
Some sites will say that because you are watching VP09 that there is no GPU support, but with vdpau installed and enabled, many GPUs will still be able to use hardware decoding for 4K........
The easiest way to know if your videos are playing with GPU acceleration are to watch the process of xplayer, mpv or whatever you are playing. The CPU usage should be no more than 10% for that process/program if it is using acceleration.
Let's manually play with vdpau to make sure it works before we make it permanent:
First make sure you have libvdpau installed:
sudo apt install vdpau-driver-all
If yo........