The dynamic increase in terahertz photoconductivity resulting from energetic intraband relaxation was used to track the formation of highly mobile charges in thin films of the tin iodide perovskite Cs<sub>1-x</sub>Rb<sub>x</sub>SnI<sub>3</sub> and compared to the lead based Cs<sub>0:05</sub>(FA<sub>0:83</sub>MA<sub>0:17</sub>)<sub>0:95</sub>Pb(I<sub>0:83</sub>Br<sub>0:17</sub>)<sub>3</sub>. Energy relaxation times were found to be around 500 fs, comparable to those in GaAs and longer than the ones of the lead-based perovskite (around 300 fs). At low excess energies the efficient intraband relaxation can be understood within the context of the Frohlich electron-phonon interaction. For higher excess energies the photoconductivity rise time lengthens in accordance with carrier injection higher in the bands, or into multiple bands. The findings contribute to the development of design rules for photovoltaic devices capable of extracting hot carriers from perovskite semiconductors.