Halide perovskites show unusual thermalisation kinetics for above bandgap
photo-excitation. We explain this as a consequence of excess energy being
deposited into discrete large polaron states. The cross-over between
low-fluence and high-fluence `phonon bottleneck' cooling is due to a Mott
transition where the polarons overlap ($n \ge 10^{18}/\mathrm{cm}^3$) and the
phonon sub-populations are shared. We calculate the initial rate of cooling
(thermalisation) from the scattering time in the Fr\"ohlich polaron model to be
78 meVps$^{-1}$ for $\mathrm{CH}_3\mathrm{NH}_3\mathrm{PbI}_3$. This rapid
initial thermalisation involves heat transfer into optical phonon modes coupled
by a polar dielectric interaction. Further cooling to equilibrium over hundreds
of picoseconds is limited by the ultra-low thermal conductivity of the
perovskite lattice.