The seismic signature of the 410-km seismic discontinuity is generally attributed to the olivine to wadsleyite polymorphic transformation. However, apparent discrepancies exist between seismic and experimental observations. Among those, the sharpness of the discontinuity as inferred from the reflectivity of seismic waves is difficult to reconcile with the gradual nature of the olivine to wadsleyite transformation predicted by phase equilibria. In this study, we explore the contribution of the phase transition kinetics to the sharpness of the discontinuity by performing X-ray diffraction and sound velocity measurements on (Mg,Fe)2SiO4 with an unprecedented time resolution as a function of the reaction progress. Our data document for the first time a transient velocity softening phenomenon and attenuation which we relate to the formation of a metastable spineloid phase. In the Earth's mantle this transformation mechanism would affect the elastic gradient within the olivine-wadsleyite two-phase loop, potentially creating a low-velocity layer; hence explaining the unique sharpness and reflectivity of the discontinuity.