A quality of moving fast, physically or mentally, is quickness. The quickness of your reaction saved your little sister from falling off the monkey bars and hitting her head.
Quickness is almost always a positive attribute, whether it's the quickness of a cornerback on a football field or an improv comic's quickness of phrase. Because it describes super speedy action, this word can also be used for a fast, fiery disposition: "His quickness of temper doesn't make him an ideal candidate for the job." The original meaning of this word, dating from about 1200, was "state of being alive."