I know that for hardened and tempered the best way is to make a treatment in molten salts.

If you have access to an ultrasonic device with sufficient driving power, and a suitable inductive heater, then you try to play with the thermal conductivity. By operating at resonant frequency, you can (for a short time) drastically alter the thermal conductivity, you can heat quickly and cool quickly. The inductive heater will transfer heat more quickly. If inductive heating is not possible then try resistive heating. Do  not bother for an insulated furnace, because most of the heat you are applying is most likely to go into the workpiece. And as soon as you stop heating, it should cool very fast. If you quench it while resonating, then you can get faster cooling.

Please, can you give more information about the purpose of your heat treatment? In fact, lot of thermal treatments are performed on HSS with different temperature, holding time and cooling methods starting from stress-relief and up to normalizing.