It's possible to make a hardware circuit, but the simplest way these days is to use a microcontroller. There are ready made speed controllers available, quite inexpensively like this one-http://hobbyking.com/hobbycity/store/uh ... oduct=8863
I'm not sure how much you know about how R/C systems work, so I'll start from the basics.
Servos are controlled using pulsed signal. This signal under normal operation works from 1-2 ms (milliseconds) with 1.5ms being centre, and the servo should swing 45 degrees each way within the 1-2ms range. It's possible to extend servo travel beyond this by increasing the range of the pulse. Most servos are mechanically limited to 180 degrees by a stop moulded in the output gear, this can be dremeled off, and the servo should then be able to move about 270 degrees- the feedback potentiometer would now be the limitation.
Under normal circumstances, you move the stick, the pulse width changes immediately, and the servo moves as fast as it can to get to the position proportional to the pulsed signal. A servo slow will have a little microcontroller (PIC Atmel etc) which will hijack the signal from the receiver, and will chop up the pulse width in little segments and put delays in between the segments.
So for instance. say you move the stick from centre (1.5mS) to one extreme of control (1.0ms). Normally the pulse would instantly change. However with a servo slow, it will go 1.5ms, delay, 1.45ms delay, 1.4ms, delay. The narrower the pulse width changes the finer the resolution. Also the longer the delays, the slow the servo moves.
If you're savvy with programming (you can use assembler, BASIC and C), then you can program this sort of thing yourself, and it's a cinch to control half a dozen servos using a single microcontroller which will only cost a couple of pounds.