¹⁹F NMR spectroscopy was used to analyze the onset of the fluorine mobility in monocrystalline LaF₃. The method is sensitive to ionic exchange with correlation times in the range of 10^-6 - 10 ^-3s. For the temperature range between 240 K and 400 K the motion is restricted mainly to the sites in the F₁ sublattice and characterized by a strongly heterogeneous behavior which can be well described by a broad distribution of correlation times. The distribution of the correlation times and its variation with temperature reflects the potential energy landscape in the superionic state. The changes in the center position and width of the distribution with temperature differ from an Arrhenius law behavior. Ionic mobility on the microscopical scale, therefore, can not be considered a simple thermally activated process.