Flashy Flash is well known for his speed but it’s not like he’s as fast as Saitama. This is particularly unfortunate for him in a fight because then he has nothing else. Saitama is also a lot more powerful than Flashy Flash although that goes without saying. That’s why there is very little chance for Flashy to do much of anything here. No matter what he tries to do, Saitama will be one step ahead of him throughout. When that ends up being the case, Flashy’s options just become very limited. Saitama wins.