I told my wife: "It's frustrating to walk behind slow walkers. There are only three types of people who walk slowly in New York: children, the mentally ill, and tourists."
Even the old and fat walk fast in New York. And the children don't technically walk slow, they just get distracted and stop and zig zag and detour.
With so little sidewalk space and so many people, New Yorkers just want efficient travel. It's a simple courtesy to pick up the pace.
My wife said: "I know, and what I hate most is walking behind mentally ill, child tourists."