  We investigate the spontaneous emission rate of a two-level quantum emitter
next to a composite medium made of randomly distributed metallic inclusions
embedded in a dielectric host matrix. In the near-field, the Purcell factor can
be enhanced by two-orders of magnitude relative to the case of an homogeneous
metallic medium, and reaches its maximum precisely at the insulator-metal
transition. By unveiling the role of the decay pathways on the emitter's
lifetime, we demonstrate that, close to the percolation threshold, the
radiation emission process is dictated by electromagnetic absorption in the
heterogeneous medium. We show that our findings are robust against change in
material properties, shape of inclusions, and apply for different effective
medium theories as well as for a wide range of transition frequencies.
