The thermal dynamics induced by ultrashort laser pulses in nanoscale systems, i.e., all-optical time-resolved nanocalorimetry is theoretically investigated from 300 to 1.5 K. We report ab initio calculations describing the temperature dependence of the electron-phonon interactions for Cu nanodisks supported on Si. The electrons and phonons temperatures are found to decouple on the ns time scale at similar to 10 K, which is two orders of magnitude in excess with respect to that found for standard low-temperature transport experiments. By accounting for the physics behind our results we suggest an alternative route for overhauling the present knowledge of the electron-phonon decoupling mechanism in nanoscale systems by replacing the mK temperature requirements of conventional experiments with experiments in the time domain.