I know that Kyros is a legendary gladiator from Corrida Colosseum who was turned into a toy by Sugar. He cuts his left foot off to save King Riku (if I'm not mistaken). But, how can he run so fast with only one foot? Even when he is a toy, he can run quite as fast and in a human body, he can take the stairs to Flower Hill faster than Luffy. How can it happen? Is he using something like Sanji's Sky Walk or CP9's Geppo?
The physics behind walking/running is:
Kyros has a much sturdier and bigger body as compared to luffy and hence the action and consequently the reaction force on him increases. Instead of running, he is actually leaping, and his leaps are consequently bigger, which makes up for the loss of his leg.
Even if people are of comparable sizes, I believe that a one legged person can run just as fast as (or only a tad bit slower than) a two legged person. The main problem that we have when trying to run on one leg is keeping our balance. Kyros has been 1 legged for a long time and hence, clearly doesn't have a balancing problem. This may also be a factor contributing to his fast speed.
You push on one foot (steps 3/4), leap (step 5), push on the other foot (step 6/1), then leap again (step 2) and back to the starting position.
If you miss a leg, you are basically leaping four steps and pushing during only two steps! No matter how sturdy you are or how big your muscles are, if you have only one leg, you can push yourself forward only half as often and thus half as fast as you can with two legs.
I think the greater contribution comes from the fact that Kyros does not have to wear the weight of the missing leg. What he misses is almost 10% of his entire full-body weight or - given his size and stature - around 9 kg or 20 lbs!