  We investigate $\mathcal{P}\mathcal{T}$-symmetric quantum systems
ultra-weakly coupled to an environment. We find that such open systems evolve
under $\mathcal{P}\mathcal{T}$-symmetric, purely dephasing and unital dynamics.
The dynamical map describing the evolution is then determined explicitly using
a quantum canonical transformation. Furthermore, we provide an explanation of
why $\mathcal{P}\mathcal{T}$-symmetric dephasing type interactions lead to
\emph{critical slowing down of decoherence}. This effect is further exemplified
with an experimentally relevant system -- a $\mathcal{P}\mathcal{T}$-symmetric
qubit easily realizable, \emph{e.g.}, in optical or microcavity experiments.
