If instant (<50ms) reaction would have lowered the speed only to 40km/h in 10m, Waymo was going too fast for the intersection IMO.

My experience is that for a human driver to react quickly in city driving conditions, style and prep are more important than reaction time: in the case you describe (entering an intersection with another car waiting on a stop sign perpendicular to your path), I'd have my foot hanging over the brake and off the gas pedal — this has helped me avoid hitting many other cars with inattentive/distracted/bad drivers, and even pedestrians running over the road or a red light on a crosswalk. When you are prepared and looking, you slam the brakes much faster!