We study the pore-scale dynamics of salt precipitation in three-dimensional drying porous media, utilizing high resolution x-ray microtomography and scanning electron microscopy. Our results illustrate that the salt precipitation patterns in drying porous media are nonuniform, manifesting the influence of the spatial distribution of pore sizes on the dynamics of salt crystallization and formation of discrete efflorescence. Results reveal that during stage-1 evaporation from saline porous media, the salt precipitation rate initially increases which is followed by a constant precipitation rate. This non-linear behaviour is attributed to the preferential liquid vaporization and salt precipitation in finer pores located at the surface of the porous medium contributing in evaporation according to the pore sizes. We also show that, contrary to common practice, the macroscopic convection-diffusion equation cannot provide accurate predictions for the dynamics of salt precipitation, at least at the early stages, due to the microscale heterogeneity of evaporation sites at the surface that results in salt precipitation exclusively in the finer pores.