For a faster 0 - Vmax (approx 2 seconds) response replace R2 by 4.7 Kohms and R4 by 180 Kohms resistor (when using 5 Kohms potentiometer). The original circuit in the Rev 001 schematic has a response time of about 3 seconds from 0 to Vmax. I don't recommend too fast settling time because of the lack of a "non-destructive" maximum current limit on the original controller. I don't like to put my luck to test doing sudden accelerations and decelerations, as it was programmed from the factory, don't compare a lathe spindle to a sewing machine's inertia.

I am also working on a PWM (isolated) speed control and On/OFF, I will be testing it in the next few days.

Note: If you are using a 4.7Kohms potentiometer keep R2 value at 4.3Kohms.