Calculations are carried out to solve the thus far puzzling problem of the long-time trends in the photoinduced condensation of supersaturated vapors observed in thermal diffusion cloud chambers. These trends are the long-time delay of the condensation after switching the light on and the long-time decay after terminating the light. The calculations show that the supersaturation level which governs the condensation process cannot be affected significantly by irradiation under conventional experimental circumstances, and hence its change cannot be the cause of the long-time transients. These trends are attributed to the slow diffusion-limited growth of liquid droplets before they achieve the observable size of 10(-3)-10(-2) cm. The predictions are in good agreement with experimental data.