After calculations of various domain-switching current transients under the pulse from electrical circuit parameters, the field dependence of domain-switching speeds is accurately estimated over five orders of magnitude in a wide temperature range of 5.4-280 K from the height of domain-switching current in Pb(Zr0.4Ti0.6)O-3 thin films. These estimations are extended following Merz's equation [W. J. Merz, Phys. Rev. 1954, 95, 690] and an ultimate domain-switching current density of 1.4 x 10(8) A cm(-1) is extracted at the highest field of 0.20 MV cm(-1). From classical domain-nucleation models with thermal fluctuations, an ultimate (asymptotic high-field) nucleation time of 0.47 ps is derived when the domain sideways motion is kink-nucleation-rate limited.