When I want to practice my swing tempo, my entire practice routine changes. Most people start with their wedges or short irons and progress to their longer clubs. However, when I practice swing tempo, I do the opposite.
I often start with a five iron or a four iron because they are more difficult to use. Most aspects of the full swing are more important with the longer clubs, including swing tempo. Naturally the swing is slower and longer with longer clubs and it’s easier for me to isolate swing tempo. You may hear commentators on television talk about players getting “quick”. This is when a player starts the down swing before he is ready. With the longer clubs, a quick transition will be much more apparent. In addition, with a longer, you’ll be able to feel the weight of the club easier throughout the swing. With a shorter club, it’s much easier to make compensations and you may not know that your tempo is off. Recently I heard that Tom Watson practices this way, too!
Alternatively, if you want to start with the shorter clubs, just make sure to focus on tempo first and consciously try not to swing too hard or fast. This will be effective as well, but you just have to be more conscious of what you’re trying to do.