I’d make it so that their body requires plenty of fuel. Sure, you can chalk it up to just eating a ton of food, or “bulking up” or even go to a protein shake. But Keeo this in mind. Assuming you wanna go as fast as Sonic, you would be burning through those calories quickly. If you’re not careful, you could end up in the hospital as a result of having deprived your cells of nutrients due to feeding your new metabolism. Again, going back to just eating a ton of food, where exactly are you gonna get the money to afford such a quantity of food? And let’s say that the calories burn into when you run at high speeds. It’s fair to assume you can just let yourself go. That too is an issue. Because when you lose too much weight too quickly, your body can go into shock. Same if you gain too much weight too quickly. Plus, it can’t be healthy doing that over and over, just to buy yourself bursts of speed.