I don't understand why my kiln fires at a progressively slower rate as the temperature climbs.

Kilns slow down as they go up in temperature. Their fastest firing rates are in the lower temperature range. The maximum firing rate of a kiln depends on the type of insulation, element design, amount of wear on the elements, the size of the kiln, the voltage available at the wall receptacle, and other factors. Ceramic fiber kilns fire faster than equivalent firebrick kilns. One of our fastest kilns is the QuikFire 6, which can reach 1000 degrees F in five minutes. The larger kilns fire much more slowly.