Desktop mode browser video playback with hardware acceleration extremely poor

[Zajko]

Steam deck standard 512 gb, docked to 4k monitor via usb-c
SteamOS: 3.5.14 preview
Firefox version 122.0
Chromium version unknown (can't turn on steam deck now)

In desktop mode, for roughly the last 1 month youtube video playback in firefox and chromium at full-hd performs extremely poorly. Issues found:
- random video freezes with only audio playing
- videos stop playing with an error
- video does not start playing for a couple of seconds when switching from another tab to the video tab
- very slow video seeking (couple of seconds per seek)
- low framerate when fullscreen (maybe 5-10 fps)
The issues get less severe, when I turn off hardware acceleration in firefox and chromium (fps is absolutely smooth, but videos still freeze up).

Hardware/ISP wise nothing has changed in the last couple of years, so I believe it is some issue with drivers/some other software updates in the last couple of months.