When I use Steam/NVidia in home streaming with my GTX 1080 card, I can render a 1080p frame in about 10-12ms and decode it in 10ms on my 4K FireTV. (I beat Witcher 3, TombRaider, and Hitman with it over a 50Mbit powerline ethernet setup and they all looked fine BTW.)
Even if the Google data center is twice as far as AWS, that's 58ms lag, or 4 frames at 60fps. You can emulate the same lag by turning off Game Mode on your HDTV, it's fine for most games.
However, it is bullshit that the Stadia website doesn't have something like fast.com that tells you if your internet connection has enough bandwidth/latency to actually work with the service.
It's also not a good sign that they're not talking about avoiding wifi for your connection the way Steam does.