Proper clamping pressure is needed to guarantee the gas-tightness of PEMFC assembly and to enhance the contacts between components. Nevertheless, excessive mechanical load can deteriorate the cell performance, particularly by reducing the porosity and the mass transfer properties of the Gas Diffusion Layers (GDLs). In this study, the global performance of a 225 cm² PEMFC assembly under mechanical stress (0.35 - 2 MPa) is investigated through Electrochemical Impedance Spectroscopy (EIS) as a complement to cell voltage monitoring and polarisation curves carried out previously. The analysis relies on the evolution, against mechanical compression, of the high and low frequency resistances, as well as the arc sizes of the EIS spectra. The measurements emphasise that mechanical loading primarily decreases ohmic losses, and they also highlight the importance of harmful physical phenomena at high frequencies. Ex-situ measurements (through-plane resistance of GDL) and in-situ results (high frequency resistance) show a similar reduction trend observed with increasing mechanical compression.