That has nothing to do with your internet speed it is just the game that is not polished yet. It is more a problem on the server end.
The video blades are recoding and encoding the video, after sending it to you. If they get overloaded or if they can't process all the information in time. Your side will get blurry.
It's like a hiccup. That is why high fps is super important for streaming services. More frames, means less blurry and less latency when the fps drops.
I am sure it will gets much smoother closer to the final release. But I still would like to play it in 4k
I also could get it on steam as well. But I wanna be able play on my tv. Or laptop. That is one of the biggest reason I am always choosing stadia over steam games. I just don't wanna pay $60 again to check what the differences are. If anyone have the game in steam and is running 4k very high settings I would like to check it.