I don't remember such a context, but if we imagine Dust Sans reaching subsonic speed, he would now have superhuman speed. Although I'm not sure the context was something like Asgore remembering something. You didn't accidentally make this up? Since you're trying to change the author's context, for example, that Dust Sans isn't interested in something, that he simply assumes he thinks so, although this is a direct response from the author, not a limitation of his power. In any case, I don't remember Kalv setting such a context, it only says that Murder flies in at a moment when the enemy is not yet prepared for battle in order to inflict serious damage. Here, yes, superhuman speed will result if we make the context that the enemy was not ready, or subsonic if this is the moment when the enemy saw Murder and should have drawn his weapon but did not have time.