It is generally accepted that DC Comics' Flash is faster than Marvel Comics' Quicksilver. In Avengers #688 though it is shown that Quicksilver can actually run as fast as the Flash, but that it is incredibly dangerous to do so. That means this legendary matchup is not as Quicksilver vs. Flash simple as everyone assumes.
These two heroes have battled for the title of fastest comic speedster for many years, most notably in the pages of DC vs. Marvel (1996), where the outcome was decided by fan voting. However, even without fans weighing in on the matchup, most metrics put Flash, (especially the Wally West version) as much faster than Pietro Maximoff. Quicksilver is amazingly speedy, but his feats tend to fall into the hypersonic spectrum, whereas Flash can actually move at lightspeeds, and even travel through time. In Pietro’s defense, it's hard for any speedster to compare to a hero like Flash that can actually lap himself in a race, but Quicksilver may not be as slow as some people think.
Related: Venom Unlocked the Jaw-Dropping True Potential of Quicksilver's Speed
During the Avengers: No Surrender story-arc, written by Mark Waid, Al Ewing, and Jim Zub, Marvel’s favorite speedster gets a power boost thanks to Scarlet Witch and Synapse. With this new supercharge to his abilities he is able to move at even more incredible speeds. Unfortunately, without any of his previous limits, Quicksilver ends up going so fast that he vanishes completely. In the DC Universe members of the Flash Family, like Wally West and Barry Allen, have the Speed Force to rely on. It’s what gives them their power and also what helps to protect them from the horrifying implications of their physics-breaking speeds. However, as it was shown
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