The eddy currents generated during the fastest disruption current decays represent the most severe design condition for medium and small size in-vessel components of most tokamaks. Best-fit linear and instantaneous plasma current quench rates have been extracted for a set of recent JET disruptions. Contrary to expectations, the current quench rate spectrum of high and low thermal energy disruptions is not substantially different. In addition, while disruptions are typically slower in helium plasmas than in deuterium plasmas, disruptions of low density helium plasmas following high current locked modes are actually faster than the fastest deuterium disruptions. The comparison between instantaneous and linear current decay rate shows that the linear approximation can significantly under-estimate the current decay rate in some types of disruptions. In these cases an exponential fit of the early phase of the current decay provides a more accurate estimate of the maximum current decay velocity. However, this fit is only suitable to model the fastest events, which have the current quench dominated by radiation losses rather than the plasma motion.