Width
Every foot is different, and comfort and performance go hand in hand. For those reasons, the width of a tennis shoe is important to achieve the best performance and prevent discomfort and injuries.
Your tennis shoes should not be too wide, as this would allow for too much movement inside the shoe, which could lead to blisters or even ankle sprains. It could also slow down your reaction time.
Conversely, if your shoes are too narrow, it could lead to poor blood circulation and a numb feeling in the toes.