Swing speed requires mostly fast twitch muscles. Obviously, you need to have muscle in the first place to hit it, but lifting too heavy of weights can actually hinder your swing and thus the speed you can generate. If you're looking at lifting weights I would recommend a program with lighter overall weights, but more reps and flexibility exercises. Someone who can hit homerun after homerun might not generate a lot of clubhead speed, just because a baseball bat is much heavier than a golf club (wooden ones, anyways) and require more brute strength. Really, it all comes down to technique. The further you can get the club to lag behind your hands, the faster your club will go as you whip it through impact. This does, however, require the strength and flexibility to be able to coil and uncoil more than any other one motion. Look at Rory McIlroy as an example: He is 5'10", and still generates 120 mph of clubhead speed. It's because his hips uncoil almost twice as fast as an average golfer, not because he has amazing strength. In fact, his hips uncoil fast enough that somewhere after impact, if you watch very high speed tapes, you will see his hips stop and actually REVERSE direction, in almost a recoil fashion, for only a few frames. That reverse is almost as fast as the average golfer comes around forward, and shows just how fast his hips are moving. They have to come back because of how the muscles connect, just so he avoids tearing them. That is an almost perfect example (in my opinion, at least) of how lagging the hands behind the club and the club behind the hands will create additional swing speed and longer drives.