If you were to watch two videos side by side of a professional golfer and an amateur moving through their back swing, you will notice the pro moves through his swing with a slow ease and sends the ball sailing, while the amateur blasts through his arc and the ball hardly goes anywhere. Slowing a back swing is essential to playing good golf worthy of a pro. Thinking "slow" is not enough to change the speed of your back swing. You have to learn how to do the action.
Stand in your normal golf stance, as if you were getting ready to hit the ball. Hold the club as if you were lining up to the tee.
Raise your club so it is held at waist height. It should be pointing out from your body.
Swing the club back and forth like a metronome. Notice how the motion is coming from the twist of your body and not the swing of your arms.
Speed up and slow down the club using the movement of your body. Listen to the sound the club head is making as it moves through the air. Experiment until you can smoothly move the club through the air in a manner in which the club head makes a moderate "whoosh" sound not a high-pitched "whish" sound.
Return to your golf stance and practice your back swing, using your body to move the swing, until you are doing so in a manner that reproduces the "whoosh" sound. That is the sound of the perfect speed at which to move through your back swing.