Electron heating up to temperatures T_e>1 keV is revealed in the cathode current-carrying plasma jet of a vacuum discharge at the current rise rate dl/dt>10¹³ A s^-1 and the pulse length tau <1 ns. It was found that the jet consists of a set of separate filaments expanding into small solid angles, which can explain the measured value of the cathode plasma temperature T exceeding those obtained earlier by more than two orders of magnitude.