Saitama's overpowered nature would completely disrupt the power balance in the MHA universe. His simple yet extreme strength and speed would make him an outlier. For example, in MHA, most heroes have to strategize and work in teams against villains. But Saitama could just one - punch most threats away. This would change the way villains operate as well, perhaps making them more desperate or more likely to target Saitama directly. It could also lead to interesting scenarios where heroes might be both in awe and a bit intimidated by his presence.
Well, Bakugo might scoff at first. He'd probably think Batman is too brooding and not as straightforward as he likes his heroes. But then as he reads more of the fanfiction, he might grudgingly admit that Batman's detective skills are cool. Deku, on the other hand, would be super interested. He'd be analyzing every move Batman makes in the fanfiction, taking notes in his head about Batman's strategies and how they could be applied in his own hero work. Uraraka would be more focused on the relationships in the fanfiction. She'd be all 'aww' at the cute moments and feel for the characters when they're in trouble.
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.