  We consider 2d QFTs as relevant deformations of CFTs in the thermodynamic
limit. Using causality and KPZ universality, we place a lower bound on the
timescale characterizing the onset of hydrodynamics. The bound is determined
parametrically in terms of the temperature and the scale associated with the
relevant deformation. This bound is typically much stronger than $\frac{1}{T}$,
the expected quantum equilibration time. Subluminality of sound further allows
us to define a thermodynamic $C$-function, and constrain the sign of the
$\mathcal T\bar{\mathcal T}$ term in EFTs.
