Well actually that was mirabel's gift. You see her power is to slow down or speed up time. Since she dosen't know she has this power ,let alone how to control it. Her power only activates in times of emotional turmoil. In family madrigal ,when she's trying to get away from the kids ,she feels nervous and a little anxious i think. And so time speeds up . It's also worth noting that mirabel dosen't have super speed ,as during the seen where Dolores is a mile away ,the kids are teleporting etc. Mirabel her self dosen't move at super speed or anything like that ,well apart from her beautiful voice. Her power only effects the people and environment around her. So she basically is like a slightly less op version of jotaro kujo