It seems studies go back and forth on this subject. Some studies show longer (3,4+ seconds) eccentric contractions are better for muscle growth while others show faster(1-2 second) eccentric contractions are better. Then others say it depends on the exercise. Personally, on heavy compound exercises I have a 3 count eccentric contraction and on isolation exercises typically go with only a 1 to 2 count. Mind you the faster contraction time is still very much a controlled contraction. I do this not based on any evidence besides my own experience with what I "feel" which is not always a good indicator of a good set or exercise but I've still been making gains. Though, as we all know, just because I've been making gains doesn't mean it's been optimal nor that it has been a direct result of the tempo I've set, it could be in spite of it. My opinion is that tempo will not be a great factor in hypertrophy but being sure to have controlled eccentric and concentric contractions is paramount.