  We present a quantitative comparison between the measurements of the complex
conductance at low (kHz) and high (GHz) frequency in a thin superconducting
film of NbN and the theoretical predictions of the dynamical
Beresinksii-Kosterlitz-Thouless theory. While the data in the GHz regime can be
well reproduced by extending the standard approach to the realistic case of an
inhomogeneous sample, the low-frequency measurements present an anomalously
large dissipative response around Tc. This anomaly can only be accounted for by
assuming a strong slowing down of the vortex diffusion in the kHz regime, or
analogously a strong reduction of the length scale probed by the incoming
finite-frequency field. This effect suggests the emergence of an intrinsic
length scale for the vortex motion that coincides with the typical size of
inhomogeneity probed by STM measurements in disordered NbN films.
