The dog is lagging because there is no consequence.
How to fix if the dog is lagging in the About Turn:

• First, we explain to the dog from where the reinforcement will be coming.
• Place him in the Basic Position.
• Have a ball under your arm.
• Drop the ball.

Then we will add Negative Reinforcement – pop the leash and drop the ball.

When making the turn, pop the leash and the dog will speed up in anticipation of the ball.

Randomise the reinforcement.