Controlled speed means hitting the center of the club face and keeping it on line.
Swinging hard generally creates off center hits and less control of the club face.
A controlled 115 mph swing struck in the center of the face will give you 315 down the middle.
A wild 120 mph swing will give you a whip hook into someone’s kitchen or a slice into a pond two holes over - and likely 40-50 yards shorter.