  In this paper we investigate the growth with respect to $p$ of dimensions of
irreducible representations of a semisimple Lie algebra $\mathfrak{g}$ over
$\overline{\mathbb{F}}_p$. More precisely, it is known that for $p\gg 0$, the
irreducibles with a regular rational central character $\lambda$ and
$p$-character $\chi$ are indexed by a certain canonical basis in the $K_0$ of
the Springer fiber of $\chi$. This basis is independent of $p$. For a basis
element, the dimension of the corresponding module is a polynomial in $p$. We
show that the canonical basis is compatible with the two-sided cell filtration
for a parabolic subgroup in the affine Weyl group defined by $\lambda$. We also
explain how to read the degree of the dimension polynomial from a filtration
component of the basis element. We use these results to establish conjectures
of the second author and Ostrik on a classification of the finite dimensional
irreducible representations of W-algebras, as well as a strengthening of a
result by the first author with Anno and Mirkovic on real variations of
stabilities for the derived category of the Springer resolution.
