The interfaces in complex oxides present unique properties exploitable in nanoscale devices. Recent studies on ferroelectric BiFeO3, BaTiO3, and Pb(Zr, Ti) O-3 have revealed an unusually high electric conductivity of the domain walls (DWs), adding another degree of freedom for controlling the local properties of these materials. While most of the investigations are focused on thin films for nanoscale applications, many practical devices, including piezoelectric sensors, actuators, and transducers, rely on the macroscopic properties of bulk polycrystalline materials where the average effect of local properties should be small. It is shown that in polycrystalline BiFeO3 the local domain-wall conductivity interferes with the dynamics of the DWs within the grains, resulting in an unexpectedly large effect on the macroscopic piezoelectric response. The results thus bridge the local conductivity and the macroscopic piezoelectricity via domain-wall dynamics, revealing that the domain-wall conductivity must be considered when interpreting and controlling macroscopic electromechanical properties.