I wonder if speed control of 50 to 30 km/h makes journeys faster in a city where you will hit traffic and traffic lights anyway. More consistent speeds, less braking.
Removing unnecessary stop lights (they will often become unnecessary when cars can just stop if there are pedestrians crossing, which is much easier from 30 than from 50) and even replacing intersections with roundabouts can make a big difference here (as long as you somehow get a population that understands what a roundabout is, anyway).