So i signed up to test how this worked under linux and the conclusion is that it's not. The lag is up to 3 seconds. how is that even possible??
I tried under windows and there you can play but under linux it is so bad I do not understand why they even allow people to try it. very very bad. I will terminate my free account its so bad I would not even play solitare in this environment.
On Linux, hardware accelleration isn't supported by default, so you are probably decoding the video stream using your CPU. That's why Linux Is still offically unsupported.
But there are some unofficial builds of Chromium with va-api support.
You can also try with Brave browser (wich is Chromium based), It should have va-api already enabled.
You probably still need to enabled the flag "override software rendering list", and you have ti enable auto playback on Stadia page
Being a Linux user I feel your pain.
As other have said before, chrome does not do hardware video decoding on linux. And you can try to get it working by using chromium. Not all gfx cards does vp9 decoding though and you might have to find a browser plugin that forces the stadia video stream to h264 if the card supports that (like h264ify).
I couldn't get chromium and hardware decoding to work properly on my setup though, so up until a few days ago I have been playing stadia without any problem with lag using the "Limit data usage" (720) setting. Visually not all that pleasing, but silky smooth. You can set this by clicking your avatar in the stadia app under "Data usage & performance". I recommend you try this setting for now.
Just a few days ago my main setup broke down so I bought a replacement mobo and cpu (A 9th gen Intel core I7-9700F) and it's fast enough to decode the video stream in both balanced and best visual quality settings using chrome on Ubuntu. Just in case upgrading your computer is an option for you.