Flash saved the population of Chongin North Korea from a nuclear blast after the blast started. Within .00001 microseconds. By carrying them out one or two at a time to a safe distance. 616 some million people. This is something absurd, but grounded in the knowable.
Imagine how fast the Flash had to be traveling. Starting in a city that he'd never been in before. Having grabbing a person or two. Running a safe distance. Returning. Finding a few more. Repeat ~313 million times. So fast that the nuclear explosion would seem frozen. After all, it, and light itself, would only expand outward three meters (3.27 yards) while this is going on.
With each save it is harder to find people. So as he beats he nears the end he has to search the city. Go into each home, reach office, each room, to make sure he misses no one in a city twice the size of Pittsburgh.
It may not be his greatest feat, but it has to be in the top five. The day the Flash didn't outrun a nuke. He ran 313 million laps around it.
