This is how I get Stadia working well on my Samsung Chromebook Pro over WiFi. If you're having any Chromebook trouble, try it!
Completely power down Chromebook.
Connect controller.
Make sure phone has a good WiFi connection, and connect it via USB.
Power up Chromebook.
Select USB Tethering on phone.
(Now the Chromebook's WiFi is being bypassed in favor of the better WiFi on the phone. The Chromebook should say in the system panel that it's connected to ethernet.)Fire up the Guest Mode on Chromebook.
(Better than just an Incognito window, because not only does this not allow extensions to run on Stadia, they aren't even running in the background using up resources.)Log into Stadia and play!
This has worked flawlessly numerous times today on "Balanced" settings, and the Stadia menu reports that the connection is "Excellent". Normally, my Chromebook isn't even able to start a game. Sometimes it can (with crappy performance and a "Fair" connection), but more often not.
What a relief! Now I can play video games in bed! 
