The flow field in a low-ns radial compressor stage is computed by means of a LES simulation performed with a commercial CFD package. Radial and tangential secondary flows are identified both in the rotor and in the diffuser. In the latter, a regime of transient stall with a blockage region oscillating between the two sidewalls is observed. Rotor secondary flows influence the diffuser regime, and some of the diffuser instabilities seem to be phase-locked to some of the unsteady large flow structures detectable in the rotor. The entropy generation rate is computed locally, to assess if and where the design could be improved. Our results confirm that entropy generation maps (both in space and time) provide designers with detailed and unequivocal information about the causes of the intrinsic flow irreversibilities.