This study provides theoretical and experimental evidence that the voltage generated in perovskite films by interference fringes' motion at low velocities can be used as a tool for assessing ions' electric mobility in these materials. We introduce a theoretical model for running fringe (RF) photo-electromotive force (EMF) in bipolar semiconductors, which considers the effects of mobile ions. This model leads to an analytical expression for the dispersion relation of space charge waves in the presence of mobile ions. Notably, it predicts a peak in the RF photo-EMF voltage when the velocity of fringe displacement matches the ion velocity in the photo-induced space charge field. To validate this, we conducted RF photo-EMF measurements on thin films of the methylammonium lead iodide semiconductor. The observed experimental dependencies align closely with theoretical predictions, and the determined values of ions' electric mobilities and activation energies are consistent with those previously reported in the literature.