The impact on the fandoms is multi - faceted. In the case of the Harry Potter fandom, it can introduce new storylines and character developments that were not possible within the original series. Fans get to see how Harry would react in a completely different universe filled with superheroes. For the Avengers fandom, it adds a touch of the magical and the complex backstories of the Harry Potter world. This can attract new fans to both fandoms. It also encourages creativity as fans try to blend the different powers, cultures, and ethics of the two universes in their own works.
Well, Saitama All Fiction could potentially refer to a fan - created idea that emphasizes Saitama's ability to be above all the normal rules of the fictional One Punch Man universe. For example, his strength is so overwhelming that it's like he exists in a realm of his own, where the typical limitations of the story don't really apply to him. He can defeat any foe with just one punch, and this 'All Fiction' concept might be an attempt to explain how and why he's so powerful in such a ridiculous way.
Saitama's speed in the manga is incredibly fast. It's hard to put a specific number on it, but he can cover great distances in no time and react with lightning speed to any threat.