The fact is that slow-speed movements in the weight room increase your overall strength and enable you to move all weights faster. For example, if your max on the bench press was 150 pounds, you probably would not move that amount of weight very fast on that lift. However, if you increased your strength to 250 pounds, you would be able to move 150 pounds a lot faster and more explosively.
If you had to choose between doing a light weight fast and a heavy weight as fast as you can (which would still be slow), pick the heavier weight. A light weight limits the amount of force you need to use, because you coast through much of the lifting phase, so you can control the weight.
Imagine doing a bench press with about 30% of your one-repetition max (1-RM). If you push the bar as fast as possible off your chest, you are strong enough to create such momentum in the first couple of inches that you would literally throw it off your chest if you applied a full effort through the full press. On the other hand, if you are training with 80% or 90% of your 1-RM, the weight is heavy enough that you have to push as hard as possible through the entire range of motion.
This isn’t to say that you should never train with high speed in the weight room. Just use equipment that can safely be thrown, such as a bag of sand or a medicine ball. (In case you were wondering, a “medicine ball” is a soft, weighted ball, often the size of a basketball.) When your intent is to throw something, you don’t need to slow it down, so you can go as hard as possible through the entire range of motion.
Throwing a medicine ball is an excellent way to train with high-speed in a chest-pressing movement. You can develop high-speed strength with a squat exercise by hugging a weight to your chest and doing jump squats.
Of the traditional weight room lifts, power cleans, snatches and overhead pushing exercises work best for high-speed lifting, although you still need a fairly high percentage of your 1-RM, so that momentum does not become an issue. Using about 75-85% of your one-rep max for these exercises will develop your strength, but allow you to move the weight quickly. You can also use dumbbells for most power exercises.