That is the good old problem for the lack of hardware video decode on Linux browsers that got worse with Stadia.
Also, first you need to make sure your VGA supports VP8/9 decode on hardware:
Since your VGA is not that old, VP9 is probably supported, so you could try this trick to make it work: https://www.linuxuprising.com/2018/08/how-to-enable-hardware-accelerated.html
For me that I have an old GTX680, no VP8/9 support at all
At least my screen is full HD only and my CPU can handle the software decode in this case.
Do note that not all gfx cards do VP9 native decoding. If you have a nvidia card, check the nvidia config for supported decoders.
(And also, if your setup is too slow to decode the stream, try setting Data usage & performance to "limited data usage" which requires lees beefy hardware)