  We study the quantum dynamics of a single impurity following its sudden
immersion into a Bose-Einstein condensate. The ensuing formation of the Bose
polaron in this general setting can be seen as impurity decoherence driven by
the condensate, which we describe within a master equation approach. We derive
rigorous analytical results for this decoherence dynamics, and thereby reveal
distinct stages of its evolution from a universal stretched exponential initial
relaxation to the final approach to equilibrium. The associated polaron
formation time exhibits a strong dependence on the impurity speed and is found
to undergo a critical slowdown around the speed of sound of the condensate.
This rich non-equilibrium behaviour of quantum impurities is of direct
relevance to recent cold atom experiments, in which Bose polarons are created
by a sudden quench of the impurity-bath interaction.
