It is worth noting that NYC and DC have fundamentally different design philosophies that result in different average speeds.

New York's system is designed around walking to stations. All the way to the end of the lines, stations tend to be about a half mile apart. There are express lines but even those become locals on at least one end, with half-mile stop spacing.

DC's is an auto-oriented system, where you're expected to drive to the stop and then take the train in from the suburbs. As a result, while there is dense stop spacing in the center, distances between stations in the suburbs get progressively longer and longer. It's over these long stretches in the suburbs that DC is able to attain much higher speeds.