Muscle tissue is composed of slow- twitch and fast-twitch fibers. Faster repetition speeds activate fast-twitch fibers and have been shown to enhance muscle power and rate of force development. For bodybuilders, activating fast-twitch fibers is most important for muscle growth. 


Performing repetitions with a slower tempo has many advantages including reducing momentum, increasing time under tension, and accentuation of metabolic stress known to facilitate muscle growth.


Although it is true that rapid-force development and muscle power are more specifically stimulated by higher rep speed, a recent review of studies comparing fast (less than one second up) versus slow rep speed (more than one second up) that also included muscle scans to measure changes in muscle hypertrophy showed that slower rep speeds may improve gains in muscle size by nearly 50% compared with faster rep speeds.


For the best gains in muscle size, slower repetition speeds come out on top.


If your main goal is muscle size, most of your training should be done using a lifting speed equal to or greater than one second up. Anywhere from one to two seconds up is about right. Any slower than that and you soon have to forfeit effective weight loads. The time lowering the weight should also fall within the one- to three-second range for an optimal growth stimulus.