Board Thread:Speculation House/@comment-4830106-20141010002549/@comment-30764484-20141010070441

About Ruby's semblance (and the same likely applies to them as a whole), she's always shown to have build up momentum to her super speed, and it also take a lot of energy. In the food fight she sort of "charged up" into it, and then with Penny she nearly couldn't get going, then passed out a few seconds later.

Taking a lot of both concentration and mental effort means it's not something she can just do all the time, like after suddenly falling through a hole.