Well they are presumably optimized for that type of attack, so they should be good at it. I get that they were going for a body horror effect, but it looked slow and clumsy and implausible. Ultimately, a fade in to the victim like they did for Izel might have worked better?
My impression is that in Yo-yo's case, she has to make a conscious decision to speed up, and that decision making process takes place in a brain running at normal speed. She can be surprised or confused and won't necessarily realize it's time for some speed before something bad happens.
The Flash has been written lots of ways in various media at various times, but I think typically his startle response will trigger his speed reflexively. (In his comic origin story he catches a dropping restaurant tray as one of his first acts of superspeed, almost without realizing it). So I'd say he has a bit of an edge in this respect.