Thing is, when I play the same video with VLC which uses HW decoding, it doesn't even break a sweat. So I think there might be something wrong on the browser part...

You can issue a bug report on https://bugzilla.mozilla.org/home Sometimes bugs are fixed really fast, sometimes never. Just give it a try.

There could be something related to FAN thresholds that resides in BIOS directly thus starting using GPU just slightly over some minimal limit might raise FAN speeds significantly and not be related to temperature raise as there is some inertia between temperature readings and real temperature inside GPU core.

I'm also experiencing FAN noise raise on my Ryzen APU based laptop.

i mostly stopped bothering with in-browser video when mpv wiht youtube-dl integration came around. either drag the url to the player window or use an open-with addon.

hardware decoding, no ads, buffering as i please

Do you have an example url for this?

Here is a bug report that shows the issue on Chrome. It is roughly 7 times slower with hardware rasterization enabled:

https://bugs.chromium.org/p/chromium/issues/detail?id=114367...

This particular example is only slow on Chrome. Even if you move the edge of the shape into the viewport Firefox is much faster than Chrome. But I know from real life examples that Firefox becomes slow if handling large SVGs. I will have to investigate this and file a bug report.

I checked again with our app on Firefox and performance is not that bad. Rendering gets a slower with large SVGs, but it is still good and there is not much difference between hardware rendering enabled and disabled. So I was either wrong about slow performance on Firefox or things have improved recently.