There are several ways to control the speed of an air cylinder.  

A) SPEED CONTROL MUFFLERS

If you are using a double-acting cylinder you can use a 5-ported 4-way valve (you may already be using one of these): 

/pneumatics/solenoid-valves/5-way-solenoid-valves.html

and speed control mufflers in each of the 2 exhaust ports:

/speed-control-muffler.html

B) FLOW CONTROLS

These control the flow of air to the air cylinder and can be used with double-acting, single acting, and reverse acting air cylinders.

There are two types of flow controls that FrightProps offers.  

In-Line flow control:

/flow-control-union.html

/in-line-flow-control.html

Male Connector Push-On with Flow Control:

/male-connector-push-on-with-flow-control.html

C) Quick Exhaust Valves

Quick exhaust valves are commonly used to increase the speed of cylinders or for rapid depressurization of tanks containing compressed air.

/quick-exhaust-valve.html