Wally… not only is he faster, I’d argue that he also has the experience. He started younger, grew with great teachers, and accomplished tasks no one thought possible (out ran existance/death pretty much to get Linda back). Current Wally has the memories of his pre-flashpoint self, so he’s not just a youngin’ compared to Barry. He’s also sitting on the Mobius Chair and Manhattan powered or whatnot. With the way he seems to be adjusting history, timelines, and potentially universes, he likely could just poof Barry out of existence or something.
As for the poster, I did think that some of the choices were odd. I didn’t expect Cheetah to be ranked so high, but I haven’t really seen the extent of her speed either. Juiced up in Bloodlines, she was a blur to WW though. As for Wallace, he’s the least experienced Flash, so I wouldn’t expect him (or any of the others who popped up around then) to compare to the other speedsters. Impulse was the only one I remember being ridiculously fast super young (not an expert there though). Bart didn’t make the cut though. I think the list was focusing on current characters (no Jay, Max, Jesse, Bart, Johnny, Fox, tornado twins, XS, Savitar, other reverse flash, Zoom, etc.), or as pointed out above many cosmic heroes. For example, Lanterns zoom all over space, and Hal was able to keep up with the speedy New God (also missing), who was outpacing omega beams. Also, WW and Shazam appear, but not Hermes/Mercury.
Personally, I think the list should have just contained speedsters to keep it simple. Premise: speed focused characters are faster than others. Then, aliens, Lanterns, and mythical/magical beings can be grouped as general families that follow the speedsters.
Edit: Although, now I want to know who the fastest super-fam member is… oh, let’s be honest; it’s Krypto.