The article cross-referenced a University of California study of average speeds in several metro areas with a survey from home improvement site Porch.com that ranked satisfaction with local transportation infrastructure. And with a 'happiness' axis on the respondents' satisfaction plotted relative to the average speed maintained in their respective areas, the result was what the analysts term a 'weak negative relationship' between speeds maintained and happiness attained. In simple terms, faster commutes can result in less satisfaction.
What the study boils down to is this: those living in metro areas with higher average speeds are combating the urban sprawl and lower density that allows those higher average speeds. You may be driving faster, but your distances are longer and – by extension – your stress (or lack of happiness) is more significant.
There is within the study's results, according to CityObservatory, an important implication in thinking about cities and how they're planned. For those attempting to navigate within them, a metro area's density and lower average speeds may be better than urban sprawl allowing somewhat higher average speeds. If that conclusion is counterintuitive, we're as surprised as you are.