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
Seems quite the same here, but only on 4K.
Been having the same issue last few days with my CCU too its like the game is lagging lol
Try turning on 50hz mode in the home app during gameplay and then turning it off again.