I've come to the conclusion that, if I'm walking at a rate of 3 miles per hour—the average human rate—Koreans are walking at about 3.5 miles per hour. This seems to hold true on flat ground, where just about everybody passes me, their short little Korean legs* working furiously to produce a per-minute step rate much higher than my own (I seem to average about 103 steps per minute). The skinnier a Korean is, the more energetic he or she is likely to be. The only people I normally pass are cripples and extremely old people. Everyone else seems somehow able to blow right by me, always in a rush to get somewhere.
The equation changes if the ground isn't level. On Namsan, for example, I beat at least 95% of the people I'm walking with, and only the craziest, most hardcore Koreans can beat me on the uphill. On Namsan, hardly anyone passes me, and I pass almost everyone.
*True: more and more Koreans are as tall or taller than I am, including Korean women, some of whom can be startlingly leggy. That said, the majority of people I encounter on any given sidewalk still tend to be significantly shorter than me.