@marjang Ah, so that could be also the cause of some additional using of the CPU.
Can you install the h264ify extension, and check if the browser uses less CPU resources when you replaying videos?
(you can see that it forces replaying videos with H.264 codec. when it displays "avc1... /mp4a..." under "Stats for nerds")
You can install the extension under a fresh profile, if you don't want to clutter the current one as follows:
Thanks @l33t4opera , it work for me!