Precise pressure measurements were carried out at phase separating the solid 3He - 4He mixtures in the temperature range of 100∓200 mK for pressures between 30∓36 bar. The relaxation time τ was found to be temperature independent at large supersaturations and/or low temperatures. This fact was explained with existence of large resistance under impurity atom transfer through the inclusion-matrix boundary. The results were compared with the τ calculate in the framework of diffusion problem which takes into account the finiteness of atom stay time on the boundary. The comparison of the calculated and experimental data allowed us to find the value of the new phase inclusion concentration. The measured temperature dependence of τ is in agreement with theoretical predictions that the diffusion coefficient must be proportional to x−4/3 (x is a impurity concentration) when there is strong impurity interaction.