How fast can a piezo actuator move?

As it is hard to define the maximum velocity of a piezo actuator, which is mostly dependant of the application, more common is to define the time it takes for an actuator to expand from resting point to maximum travel range. This is referred as rise time (T) which depends on the is the resonance frequency (Rf) of the piezo actuator defined as

T = 1 / (3 * Rf)