If he hears the command he is allowed to take the obstacle. The dog should get to the point where he will do this off leash. Go back to the obstacles and begin training off leash. Basically you are telling the dog if you don't play the game the way I say, you will be put in a down and the game is over. This is often more of a control issue than a speed issue.
Many speedy dogs have control.
Keeping an excited dog focused on the work at hand is another problem. When the situation deteriorates into the dog running circles around the handler the dog is no longer focused. It's on an adrenaline rush, and this isn't limited to herding breeds. Teaching the dog to look ahead for the next obstacle helps. Step back to 3 jumps in a row.