Why are planes slower than they used to be?

It's a fact that the more aviation savvy among you may have already observed: on the whole, airliners fly slower than they used to. If we take a random flight, for example from New York to Denver, it takes 19 minutes longer today than in 1983 to connect the two cities. This seems to go against the grain of technological progress, so what are the factors that explain this?


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