I also was a participant. It was pretty cool. I did a wireshark and they are using Google's QUIC protocol.
In my experience the game would drop graphics before I would experience input lag. There were a handful of times that I did experience input lag. This was on a wired connection 100mbs down 20mbs up through Xfinity.
I did notice in the Google demo, that when the person was using the game pad, he experienced input lag when he was trying to jump up on top of that steeple on top of the building.