[en] The LHCb Muon System is composed of five detection stations maninly equipped with a total of 1368 Multi-Wire Proportional Chambers. In order to allow a fast evaluation of the transverse momentum of the muon, the detectors have to provide a fast response and a high efficiency. We present a novel precise study of the performance of different chambers, completely equipped with the final Front-End Electronics, measured with cosmic rays. Measurements of the time and space resolution have been carried out as a function of the gas gain and discriminator threshold along with detailed studies of the time and space structure of the cross-talk hits. The studies perfomed are usefuls to get a better knowledge of the behavior of the chambers and to optimize the working conditions on the apparatus during the LHCb data taking