Hey - it's "mystic" Krillin. How can a normal human reach those high, high pls? By magic. Mystic Gohan's unlocked powers don't neccessarily stem from his saiyan born nature, after all.
If that doesn't satisfy you, then that's too bad. It doesn't have to make "sense", anyway. also, Krillin probably won't be the fastest once the other characters get their other transformations.
Krillin won't (or shouldn't) be replaced because not only does he have a good list of moves, but there's no good reason too. The little bald guy played more of a role in DBZ than Gotenks, as well. Give him some credit.