Understanding the non-equilibrium dynamics of quantum manybodysystems is of fundamental importance. We have used an interferometric method to study dynamics of spin impurities interacting with an atomic BEC and resolve the ultrafast processes occurring on the degeneracy time scale. Our approach grants access to the quantum coherence of the impurity. We observe an initial universal evolution, governed by unitary limited scattering between impurity and medium particles. For weaker interactions, we find a dynamical crossover from the universal behaviour to a weak coupling regime. Despite the complexity of the system, both these regimes are described by simple power laws of different exponents. In addition, we observe the emergence of manybody dynamics signalling the onset of polaron formation. We compare our experimental data with rigorous results valid for short time or in the weakly interacting regime as well as with a non-perturbative theory for strong interactions obtaining excellent agreement.