The elbows stay together and in front of your body throughout the swing. About waist high in the backswing the right arm will hinge. If you keep your elbows together through the hitting area, the left arm will rotate, the right arm will extend and cross the left producing the crossover. This is the power. For our purposes, tempo is the length of time it takes to make a swing from start to finish.
Speed is how fast the club swings.
You must have a smooth, rhythmic tempo to get the most out of your swing. Not fast, not slow, but smooth and rhythmic. Try to swing all your clubs with the same tempo. The speed of the club is dictated by the length of the club and should not be your concern. The same tempo is your concern.