We study the dependence of the magnetocaloric effect on the magnetic field-change-rate the first order magnetostructural transition in Mn₃GaC by measuring the adiabatic temperature change ΔT at three different time scales: 11 mT s^-1, 700 mT s^-1, and ∼1000 T s^-1. We find that the maximum adiabatic temperature-change of about 5K is reached in the 11 mT s^-1 and 700 mT s^-1 rates, whereas for the ∼1000 T s^-1-rate the transition lags the change in the magnetic field so that the maximum adiabatic temperature-change is not attained.