My laptop is usually noisy when the CPU is in demand. I am a bit surprised because playing HD videos on YT does not make the fan too noisy (comparing to loading a website with a ton of JS,for example).

So, I concluded that some HW decoding was in use, somehow. I can, of course, be wrong.

There is no way that ffmpeg uses a HW decoding decoder under the hood? It may explain why it's so smooth. Besides, when I run VP9 video with MPV, it tells me that it is explicitly using HW decoding via vaapi. And MPV is based on ffmpeg libs for the playback, if I am not mistaken.

Thanks for your light!

Although it's possible to utilize hardware decoders with FFmpeg, frames need to be shown somehow. Downloading decoded frames back to CPU memory just to copy them back to GPU to display is not a good idea. Generally graphics adapters expect a lot more data coming to them, not from them. It may be so slow, that it makes the whole idea infeasible. To make hardware decoding actually useful, one needs to figure out how to combine everything together on GPU side. It's harder to do it in Firefox than in a video player, since browsers have a complex rendering model. And now even run in multiple processes.

> My laptop is usually noisy when the CPU is in demand.

Not only software codecs are highly optimized, CPUs are also power-efficient. 1080p VP9 decode on my Skylake laptop doesn't produce enough heat to start the fan.

Out of curiosity how do you check that? I'm asking because I'd like to check it on my end.