I heard Raymond Floyd at an exhibition say when he wanted to hit it farther he swung slower. This allowed him to make a bigger turn. When trying to add speed you typically will swing faster which will tighten muscles up, shorten the swing and throw off the timing.
There are several good speed training aids on the market today. Driver length shafts of different weight with no club head on them. They are really good at helping increase speed by following their program.
Fitness training helps too. Leg and core workouts and proper stretching.