Here's the wiring for the DPDT reversing switch:-
simulate this circuit – Schematic created using CircuitLab
As you can see, a separate wire supplies +12V for forward and reverse. So to make the the actuator go slower in one direction you just have insert something in one of these wires to drop the voltage.
The actuator probably has a DC motor inside. You could use a resistor to slow it down, or several different resistors chosen with a multiway switch. However the motor will draw more current as loading increases, causing the resistor to drop more voltage and slow it down even more. If the load is constant then you can just try different resistor values until you get the speed you want, but if it is variable then actuator speed will vary greatly, possibly even stalling if the voltage drops too much. If the actuator draws a lot of current then the resistor will get hot and needs to be a high power type.
A PWM motor controller solves these problems because it doesn't use resistance to drop the voltage, but instead rapidly switches the motor fully on and off with a variable ratio. The motor sees this as an average voltage that doesn't change with loading, and the controller stays cool because it is either fully on or off and so doesn't waste power.
The power supply can be wired directly to the controller, but its output may not be referenced to either power rail so to get full speed in one direction and controlled speed in the other you should switch both motor wires. The circuit would look like this:-
simulate this circuit