  Time-dependently driven stochastic systems form a vast and manifold class of
non-equilibrium systems used to model important applications on small length
scales such as bit erasure protocols or microscopic heat engines. One property
that unites all these quite different systems is some form of lag between the
driving of the system and its response. For periodic steady states, we quantify
this lag by introducing a generalized phase difference and prove a tight upper
bound for it. In its most general version, this bound depends only on the
relative speed of the driving.
