In Season 2 episode 2 Saitama showed to Speed of Sound Sonic he was much faster than him. Speed of Sound Sonic is supposed to be a fast character, faster than Genos with its last upgrade. In the show is clearly shown that Saitama is the strongest character, but how about his speed? Is he also the fastest character in One Punch Man or are there characters which are equally or faster than him?
Without specific spoilers, the short way to put this is that Saitama has proven himself effortlessly faster and stronger than everyone he encounters, including those specifically known for being faster or stronger than everyone else (except Saitama; and Blast has not been seen in real action, and certainly not relative to Saitama).
Spoilers of certain specific events from the webcomic:
Late in the Monster Association/Garou saga he encounters Flashy Flash in some tunnels. Flash attacks him at high speed, assuming Saitama is a monster to dispatch. Saitama effortlessly dodges. Flash is shocked, and decides to try it again and makes another high speed (sneak) attack. Saitama simply stops the blade between his fingers.
Flashy Flash then later fights against Garou. Garou proves himself faster than he can handle. Saitama effortlessly keeps up with Garou's pace when they start fighting, however. Just when Garou thinks he's snuck into a blind spot, suddenly Saitama's eyes will shift to look directly at him, and Garou will back off, frustrated that he can't exploit any of the many openings Saitama has because he's simply not strong or fast enough to do so. I believe Garou has been stated by ONE to be comparable to Boros in overall power, though Garou fights primarily with skill and speed while Boros fought with sheer power. Saitama could effortlessly handle both of them.
As I've mentioned in another Q&A:
Saitama appears to be without limits. Dr. Genus talks about how every being in existence has an innate limiter on their power and growth, because beyond that they'd physically or mentally collapse. With science, training, monsterification, etc. at most you can increase what your limit is. But you'll still have one. Saitama, on the other hand, removed his limiter entirely. If he's correct, then Saitama is arbitrarily strong and arbitrarily fast. He just needs a reason to actually use more strength or speed.
According to this astrophysicist, to destroy a meteor like Saitama did (yes, he explicity mentions Saitama and the meteor Saitama destroy) you would need to travel to 99.99999997% the speed of light. So Saitama moves as fast as the speed of light, and no one can be faster than him in One Punch Man since the speed of light is the fastest known speed in the universe.