Recent numerical simulations show that elastic strains in mixed-halide perovskites play a decisive role in the kinetics of light-induced spontaneous ion demixing, which deteriorates the performance of the perovskites as photovoltaic elements. The role of elastic deformation is investigated in detail, taking in to account a dependence of elastic constant on local chemical composition. The results of the simulations indicate that ion demixing is possible only if it is not accompanied by an increase of elastic energy (demixing on grain boundaries or other structural defects)