An exceptionally high stimulated Raman scattering (SRS) conversion efficiency to the first Stokes component associated with the secondary (low-frequency and low intensity) vibrational mode ${\nu }_2$ ($\sim 330{\mathrm{cm}}^{-1}$) was observed in a ${\mathrm{BaWO}}_4$ crystal in a highly transient regime of interaction. The effect takes place in the range of pump pulse energy from $\sim 0.1$ to $\sim 0.5\mathrm{\mu J}$ with maximum energy conversion efficiency up to 35% at 0.2 µJ. The nature of the observed effects is explained by interference of SRS and self-phase modulation, where the latter is related to a noninstantaneous orientational Kerr nonlinearity in the ${\mathrm{BaWO}}_4$ crystal.