Most bodybuilders when they first start weight training to develop more muscle seem to forget the key reason why a muscle grows which is time under tension (TUT). If you walk into any gym and look at the average speed that people are going through their sets and reps, it will be faster than ideal.
The writer of a leading bodybuilding book called "Optimum Anabolics" called Jeff Anderson, says that slow reps for maximum muscle gain results should take 10 seconds each. The concentric phase or the positive phase should be 4 seconds with 2 seconds at the top while squeezing the muscle a bit more and then followed by a 4 second eccentric phase to lower the weight.
It is ironic that most people who start training seem to follow a speed that completes a rep in about 2 or maybe 3 seconds using a fast explosive movement doing the concentric phase. These are the essential ingredients for strength gain and not muscle gain according to Jeff Anderson.
The irony is that most bodybuilders are fully aware of TUT and that the reason a muscle grows in size is directly related to the amount of time is spent under tension. Training by explosively lifting the concentric phase and lowering the weight eccentrically for 2 or 3 seconds is designed to increase strength, not size.
The results are that most bodybuilders fall into the strength trap, which means they see that the more weight they can lift the more muscle they get. But the argument from experts like Jeff Anderson suggests quite emphatically that this approaches the objective of muscle gain completely incorrectly.
Jeff Anderson explains that a well-performed slow repetition will take 10 seconds as mentioned above. Whether we train with fast or slow repetitions the muscles will be stretching and contracting during the movement creating small microscopic tears in the muscles fibers causing delayed onset muscle soreness (DOMS) for 24 or 48 hours after exercise.
Muscles are built during the reparation phase while DOMS starts to peak and then slowly fade away as the muscles develops protection from the same thing happening again by getting bigger and stronger. This is why hydration and eating protein after a workout to help the protein synthesis to build the muscle is so vitally important.
In conclusion training with a fast tempo, fast reps' workout is recommended by the experts only in cycle, but more importantly it means to accent the explosive lift of the movement, which will enable you to lift a heavier weight. This in turn will boost your core strength and your power.