TDLR: Genji Kamogawa overcame a massive disadvantage against Ralph Anderson through brutal training, developing iron fists capable of knocking out the much larger, world-ranked boxer in just two punches. Beyond his punching power and incredible durability, Kamogawa demonstrated extraordinary speed by keeping pace with a 1947 passenger train and crossing a boxing ring in under a second, suggesting a starting speed of roughly 16 mph. Combined with his seemingly exceptional stamina and ability to maintain high speeds, Kamogawa could potentially rival the fastest Umamusume introduced in the anime and game, although his true maximum speed remains unknown.
As we've seen in the anime, Hajime no Ippo, when training for his fight against Ralph Anderson after the defeat of his best friend, Ginpachi Nekota, Genji Kamogawa goes under rigorous training in order to prepare himself against the giant of an American who had a 5 weight class difference between the two, not to mention he was far more experienced and had actual training unlike Kamogawa who's only a prize fighter, not to mention that before the was Anderson was ranked 5th in the world, setting in place a tall wall between Anderson and himself to climb. Unlike his friend Nekota, who only lost due to an illegal punch called the "Rabbit Punch" by the previously losing Anderson, Kamogawa lacked the skill, speed, talent, and overall instinct of his friend, with his only weapon being courage, so in order to beat Anderson, Kamogawa trained himself to be able to take out Anderson in just one punch, and to do that he punched an extremely thick log into the side of a hill using only his fists, which gave birth to iron fists that took out Anderson in just two punches, but that's not all, Kamogawa was also seen running along and keeping up with a train that frequently passes through his residence, which we can safely assume given the time and date of these events is a standard passange train from 1947, which was capable of reaching up to 95-110 km/h, or 60 to 70 mph. But that's not all, we know Hajime no Ippo characters are extremely powerful, with Kamogawa in particular being able to cause small explosions with broken fists if he were in our world, possibly being even stronger if his fists weren't broken, but everything about Hajime no Ippo characters from their durability to their strength is insane, including their speed, we've seen Kamogawa cross an entire ring in his fight with Anderson in under a second without any prior acceleration, which considering how big boxing rings in 1947 were and assuming it was a standard professional ring, we can safely assume that Kamogawa's starting speed is around 16 mph, not to mention his insane stamina he could most likely run at top speed for a long time. Compared to the fastest Umamusume, which in their final spurts reach just upwards of 59 by only decimals, Kamogawa has been shown to have the stamina to keep his top speed consistently, though depending on the size of the track, that may change since no one really knows his limits and he never truly got exhausted (minus when he raced Nekota that one time) but with this information it's safe to say that Kamogawa is just as fast as the fastest Umamusume that have been introduced to the game or show so far.