  Structural martensitic transformations enable various applications, which
range from high stroke actuation and sensing to energy efficient magnetocaloric
refrigeration and thermomagnetic energy harvesting. All these emerging
applications benefit from a fast transformation, but up to now the speed limit
of martensitic transformations has not been explored. Here, we demonstrate that
a martensite to austenite transformation can be completed in under ten
nanoseconds. We heat an epitaxial Ni-Mn-Ga film with a laser pulse and use
synchrotron diffraction to probe the influence of initial sample temperature
and overheating on transformation rate and ratio. We demonstrate that an
increase of thermal energy drives this transformation faster. Though the
observed speed limit of 2.5 x 10^{27} (Js)^{-1} per unit cell leaves plenty of
room for a further acceleration of applications, our analysis reveals that the
practical limit will be the energy required for switching. Our experiments
unveil that martensitic transformations obey similar speed limits as in
microelectronics, which are expressed by the Margolus-Levitin theorem.
