At least he said he could go all day just using the insane speed boost. Maybe the heavy attacks he used against Youpi were too much. Also, though I'm repeating myself, he's still a kid. How long has he trained his electro attacks? Months? A year at most. And nobody can help him with it, since most people wouldn't be able to handle it.

I actually like it more that they would get tired instead of going forever like a duracell bunny. It's far more human and makes fights more exciting. The arc is already losing it, or lost it, with the King becoming a god. The last thing we need is all the others becoming mini-gods as well.