I was in the project stream beta and the latency was never noticeable for me. I have seen noticeable latency on TVs not set to game mode when playing on xbox, but did not see any streaming latency for the beta.

I was on a cable internet connection and the display occasionally pixelated during network slowdowns, but there was still no latency noticeable.

I assume that Google has enough data center locations that they can render you at the nearest one.