The response is normal that you saw.
Think of it this way: when the shocks are on the vehicle the main thing they do when extending is to damp the suspension dropping away from the vehicle by it's own weight, or the body being pulled up by a wheel at the opposite corner going down.
In both cases, the shock will be "feeling" the weight of the suspension on that side of the truck.
When you release it on just it's gas pressure, there's not enough force to make it move fast. It's doing exactly what it should and it's an indication of a good shock more or less. The only way to really tell is to see how it responds to a certain amount of force to verify it's valving -- but what you feel with it being pressurized and the slow movement when you release it sounds good.