Some personal trainers recommend training at a slow tempo because they believe that prolonged tension promotes muscle hypertrophy and strength. A study from Springfield College in Massachusetts found that training at a faster tempo (two seconds down, two seconds up) increased performance in the bench press better than a slower tempo (four seconds down, two seconds up).
Training in a faster tempo resulted in the greatest increases in IGF-1, which is an important anabolic hormone. Training at a slower tempo reduces the capacity to lift heavier weights, and will result in reduced training gains. (Journal Strength Conditioning Research, 25: 406-413, 2011)