  We present results from the first multi-epoch X-ray/IR fast-photometry
campaign on the black-hole transient GX 339--4, during its 2015 outburst decay.
We studied the evolution of the power spectral densities finding strong
differences between the two bands. The X-ray power spectral density follows
standard patterns of evolution, plausibly reflecting changes in the accretion
flow. The IR power spectral density instead evolves very slowly, with a
high-frequency break consistent with remaining constant at $0.63 \pm 0.03$ Hz
throughout the campaign. We discuss this result in the context of the currently
available models for the IR emission in black-hole transients. While all models
will need to be tested quantitatively against this unexpected constraint, we
show that an IR emitting relativistic jet which filters out the
short-timescales fluctuations injected from the accretion inflow appears as the
most plausible scenario.
