I used to play with Stadia 5 months ago, and it was close to perfect.
I now returned to using it today and it became so bad, lags for 0.5-1 second every 4-5 seconds, I'm on a good and stable 70/18 MB DL/UL
I tried everything, did the recommended test on the website and it said your connection is good and you should see high quality ... etc
I tried it on Chrome Mac / Chromecast Ultra.
I also tried to set res. to 720p / 1080p but not difference, the stream quality is very good, but then it lags/freezes for 100-500 ms every 4-5 seconds
I'm in London / UK