A slow, steady, and controlled tempo is safer and best for stimulating muscle hypertrophy.
Lifting too slow reduces the amount of weight that can be used and thus limits ability to add weight and make continued gains.
It is speculated that slowing the tempo reduces momentum, thereby increasing tension on the muscle.
Increasing the tempo allows greater loads to be used, potentially increasing the anabolic stimulus.
Lifting weight using a quick tempo is believed to put the lifter at risk of injury.
When lifting speed is increased, form is often sacrificed.
Studies comparing different rep tempos have found no significant difference in hypertrophy between .5 seconds all the way up to eight full seconds.
Growth would appear to be similar when using rep speeds anywhere from .5 seconds to eight seconds per rep. The primary stimulus for growth during a set is not the speed with which the muscle contracts.
When using lighter loads, slow down the tempo to increase metabolic stress, which you can gauge by the burning sensation. As the weight loads get heavier throughout your training cycle, rep tempo can increase to ensure successful completion of each set. - FLEX