When I watch a golfer hit a 7-iron, then a driver, he or she invariably amps up the swing speed with the longer club. Surely, the clubhead of the driver moves faster because it's longer, but it's because of the principles of physics, not because the golfer is swinging the club with a faster tempo.
Tempo is the total amount of time it takes to create your golf swing from beginning to end. Some players have a relatively fast tempo, like Nick Price, while others have a slower tempo, like Fred Couples. Either way is fine, as long as you keep the same tempo for each club in the bag. Golfers get into trouble when they either slow down or speed up their natural tempo.
When your tempo starts varying from club to club, the timing required to hit consistent golf shots is destroyed. That's one reason why you feel you can hit your irons well one day, but not the woods, and vice versa. For every club in the bag, the tempo should be the same. It should take the same amount of time to make a swing with your pitching wedge as it does for the 7-iron and the driver, for example. What's different is the speed of the clubhead. Because the driver is longer than a wedge, the clubhead moves faster throughout the swing, but if it takes two seconds to swing a wedge, it should take the same two seconds to swing the driver.
Discover your ideal tempo by making three continuous practice swings, without a ball, using a 5-iron. Make the swings in a pendulum fashion, back and through while maintaining good balance. Then hit a teed golf ball, focusing on repeating the same tempo with a balanced finish. Perform this drill with short irons, long irons and woods as well to see if your tempo–and timing–remain constant.