Not familiar with Spectrum, but my Futaba has the clever option of bypassing the switches and using the joysticks that can be moved at either fast or slow speeds and in small increments if desired.

Seriously, you can either program in exponential so the response is "dulled" near the center of the throws or you can adjust the endpoints so you are moving through broader or narrower arcs in a given time. You can also effectively slow a servo by moving in on the servo arm and out on the control horn. But short of adding a resistor to the servo circuitry you cannot change the motor response time. Would that we could speed them up sometimes.