I would suggest a standard valve similar to #4995K23 shown on page 427 of http://www.mcmaster.com/ Install in the hose that connects to the base ( opposite the rod) of the cylinder. You can then adjust it to the speed you want and keep the "T" handle in your pocket to prevent tampering. This valve will control the flow in both directions. The one above has 1/4 pipe thread so you may need some adaptor fittings.

If you use the flow control valves shown on the same page you will need two of them, one on each hose going to the cylinder, as they only restrict flow in one direction and they need to be installed so they restrict the flow going into the cylinder.