  We study the Fr\"ohlich polaron model in $\mathbb{R}^3$, and prove a lower
bound on its ground state energy as a function of the total momentum. The bound
is asymptotically sharp at large coupling. In combination with a corresponding
upper bound proved earlier, it shows that the energy is approximately parabolic
below the continuum threshold, and that the polaron's effective mass (defined
as the semi-latus rectum of the parabola) is given by the celebrated
Landau--Pekar formula. In particular, it diverges as $\alpha^4$ for large
coupling constant $\alpha$.
