More useful speed starts with movement you can control. A golf-specific strength plan should support a repeatable swing, rather than chase numbers at the expense of it.
Screen the movement
A Titleist Performance Institute (TPI) thoracic mobility screen can help a qualified coach identify how comfortably you rotate through the upper back. Pair the screen with an assessment of hips, balance and your actual swing. A limitation is a starting point for coaching, not a diagnosis.
Build the posterior chain
Learn a stable hip hinge before adding load. Begin with a dowel or light kettlebell, progress to a controlled Romanian deadlift, then consider a trap-bar or conventional deadlift when technique and training history support it. Leave repetitions in reserve, train consistently and allow recovery between demanding sessions.
Turn strength into speed
Medicine-ball rotational throws can teach fast force production: start with a light ball, brace comfortably and rotate through the hips and torso. Use a suitable wall and plenty of space. Combine a few quality throws with mobility and supervised speed work rather than grinding through fatigue.
Natural off-season hypertrophy comes from progressive training, sufficient food, protein and sleep. A goal of adding 6–9 mph of driver ball speed may be achievable for some players, but it is not a promise: baseline fitness, technique, equipment and practice all influence results. Consult a qualified professional, particularly if you have pain or an injury history.