I don't think that CPU is the issue... I have an all-AMD setup (Ryzen 5 1400 for CPU, RX 550 4 GB for GPU) that I build myself back in 2017, and had no stutteting issues on YT.Interesting. I have a (relatively) new shiny Threadripper with an Nvidia GPU that has constant stuttering issues on YT, both Firefox and Falkon.
Are you saying this is a CPU issue? Is there anything I can do about that? Because I was scratching my head over this like OP, and just assumed that it probably had something to do with the unlocked functionality of the Nvidia.
(Not that I specifically went to check for that, but with plenty of embedded YT vids all over the Internet, some of 'em actually useful in the moment, it was an unavoidable, in-your-face fly-by, and I had no issues at all with loading or stuttering in Konqueror that I compiled from ports with everything enabled).
I guess compiling multimedia/ffmpeg with EVERYTHING enabled (even for deps of deps of deps of deps) proved useful. Pre-compiled packages (the ones that get installed with
# pkg installtend to be compiled with minimal options. When proper codecs are not installed (or compiled without usable options), video playback suffers, and even a good GPU will be unable to compensate for that.