By applying high electric fields to triglycine sulfate at temperatures close to the Curie point, switching times of the order of nanoseconds were observed. It was assumed, from previous studies, that at these high fields all domain nuclei form almost at the same time and grow together through the thickness of the specimen. In this case the transit time for the domain nuclei is almost equal to the switching time and the experimental results show that the domain velocities observed in the faster switching times were supersonic. A sudden increase in wall mobility occurred at the transition from subsonic to supersonic velocities. Both the supersonic and the subsonic mobilities are thickness‐dependent. Models to explain these effects in a qualitative way are provided.