This feat is annoying to quantify. There are very few reference points, and the camera is always moving to make it look like things are still frozen in the air.
But, there is one spot of hope here. Around the 2:15 mark, when Superman jumps to crush Barry, the scene briefly shifts back into real-time, showing us how fast Wonder Woman is actually moving through the air. If we can find out how fast that is, and how fast she appears to be moving, then we can determine Barry's speed in this scene.
But here comes another problem, the camera turns with Wonder Woman to make it seem as if she isn't moving.
However, there's one solution to this as well. If we assume that Wonder Woman and Aquaman are on the same plane of movement, then by determining how fast Aquaman appears to be moving as the camera turns will tell us how fast Wonder Woman must be moving to appear still during that turn.
So let's get to work on that.
Aquaman is played by Jason Momoa, who's 1.93 m tall. In this scene he's about 159 px tall, and moves a horizontal distance of 39 px over 1 s. This translates to him having crossed 0.427 m, for an apparent speed of 0.427 m/s.
Wonder Woman is played by Gal Gadot, who's 1.78 m tall. In this scene she's 275 px tall, and she moves about 183 px horizontally, with some room for error due to the blur and slight horizontal camera motion. This translates to her moving around 1.18 m over this 0.12 s timeframe, for a speed of 9.83 m/s.
Since earlier we established that the apparent speed was 0.427 m/s, this would mean that the scene is playing 23~x slower than real-time. So, what does that mean?
Well, let's assume Barry is keeping a running pace of around 4 m/s here. That'd be boosted by 23x, making him actually run around 92 m/s.
It takes Superman about 0.2 s to throw a punch, meaning he's actually throwing his punches every 0.0086 s. If he throws his punch over a distance of about 0.5 m, that gives him a punching speed of 58.1 m/s in this scene.
So on so forth. Practically everything in this scene is moving fast enough that the world would seem to be in, well, super slow motion.
Results
- Barry Allen Zips Around (Slow Motion Factor) - 23~x slower than real-time
- Barry Allen Zips Around (Barry's Running Speed) - 92 m/s
- Barry Allen Zips Around (Superman's Punching Speed) - 58.1 m/s