There is little debate that in the DC universe, the Flash is the fastest man alive - he’s even had multiple races against Superman to prove it. However, there is one hero who can give the Scarlet Speedster a run for his money: Captain Atom.
Captain Atom was originally created for Charlton Comics back in 1960, but after the company was bought out by DC, the character was reimagined. The DC version of the character’s real name is Nathaniel Adam. Nathaniel was formerly a pilot during the Vietnam War, and after being framed for a crime, was given a choice: execution or participation in a nuclear experiment that would, in all likelihood, kill him as well. Nathaniel chose the latter, better a 1% chance of survival than none at all. Nathaniel was seemingly disintegrated, but reappeared years later with superpowers and operated under the name Captain Atom.
The New 52 saw Captain Atom get rebooted yet again, though the basics of his origin story stayed the same. He was still an Air Force pilot who participated in a nuclear experiment that gave him superpowers, which include super strength, flight, energy projection and absorption, and atomic transmutation. However, while Captain Atom wants to use his powers for good, he doesn’t fully understand them yet. Not only that, but he’s essentially made of nuclear energy, meaning he could present a great danger if he ever became unstable.
Much like Doctor Manhattan - who, according to the author of Watchmen, was actually inspired by Captain Atom - Nathaniel’s perception of time doesn’t work like a normal person’s. In Captain Atom #3, titled “Divine Intervention,” he says, “Time stretches — hours, minutes, seconds. They start to lose their meaning altogether. My world is in between the one everyone else lives in.” Captain Atom's slowed-down perception of time makes him feel alone - that is, until he comes across the Flash.
The Flash also has a unique perception of time, which slows down tremendously when he activates his super speed. When Captain Atom shows up to assist rebels in Libya and finds the Flash already there helping out, the two have a long conversation that actually only takes place over a few milliseconds. So while we don’t actually see the Flash "race" Captain Atom in the same way he does with Superman, we know that the two can perceive time at the same pace - and that’s arguably even more impressive.