You need something to dampen the spring action of the coil spring, as there are no 'slow return springs' that I am aware of. The inherent quality of a spring is that it 'wants' to return to its free length as soon as the potential energy it has stored is allowed to flow.
Typical method would be to couple the spring to a hydraulic cylinder with a small passage through the piston to allow the cylinder to 'leak' oil from one side to the other at the desired rate. A shock absorber or gas strut may work in similar fashion.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)