The theory of open quantum systems addresses how coupling to external degrees of freedom
modifies observables and quantum coherence, a situation central to fundamental condensed-matter
research and emerging quantum technologies. Schwinger-Keldysh field theory is a natural framework
for both open- and nonequilibrium quantum systems in terms of functional integrals. However, its
numerical solution is limited by a cubic scaling with the number of time steps. This is particularly
prohibitive for scenarios with widely separated time scales, as is often the case for system and
environmental scales. We consider a damped quantum harmonic oscillator as a toy model to study
a separation-of-scales ansatz based on Hadamard regularization. A time-stepping algorithm for
the Kadanoff-Baym equations on the slow system time-scale is presented that captures both low-
temperature non-Markovianity and renormalization effects arising from the much faster environment
scale.