Which of the following is NOT an effect of grip size on performance?

The influence of grip size on performance can be significant in several areas, but controlling shaft flex is not one of them. Grip size affects how a player holds the club, which can influence their perception of feel and comfort, thus impacting their ability to swing the club effectively. A proper grip size can enhance a player's swing speed and contribute to shot accuracy by promoting better positioning of the hands during the swing.

However, shaft flex is determined primarily by the material and design of the shaft itself, not the grip size. The flex of the shaft influences how the clubhead behaves during the swing, affecting trajectory, distance, and control, but it operates independently of the grip size. Therefore, the notion that grip size controls shaft flex is incorrect and clarifies why this option stands out among the others regarding the effects of grip size on performance.