In this work, we combine conventional linear response time-domain THz spectroscopy with non-linear THz-pump THz-probe techniques to study metallic strained thin films of ${Ca}_2{RuO}_4$, which undergo a transition into a ferromagnetic state at . Such measurements allow us to independently measure the momentum and energy relaxation rates. We find that while the momentum relaxation rate decreases significantly at the ferromagnetic transition in accord with the usual resistive anomaly near the Curie temperature, the energy relaxation rate remains unaffected by the emergence of magnetic order. These observations show that the dominant changes to scattering across the transition correspond to scatterings that relax momentum without relaxing energy. This behavior is consistent with a scenario where energy is not carried off by coupling to collective magnetic degrees of freedom. Instead, the principal channel for energy relaxation remains the conventional one e.g. coupling to acoustic phonons. Our experiment then validates the approximation used in the conventional understanding of resistive anomalies of ferromagnets across the Curie temperature, which due to critical slowing down, spin fluctuations can be treated as effectively static and scattering off of them elastic. This scenario can likely be extended to resistive anomalies at other phase transitions to charge- and spin-density wave states in kagome metals or pnictide systems.