When people hit the gym every day they want to maximize their benefits with the time they are investing. Being efficient seems to make sense when you have specific goals in mind. For some it is strength while for others it is bulk. Fast or slow repetitions will make a difference in which goal you actually achieve.
Fast Repetitions: Faster repetitions will get you strength. This is helpful for those who seek to create speed and strength for sports such as boxing, running, basketball and others that require fast twitch muscles.
Slow Repetitions: Slower repetitions will afford the opportunity build bulk. If you are looking to create a strong physique and seek to look "fit". As the slow down the repetition you are engaging slower fibers and creating more tears.
Which is better?
It depends on what you want strength & speed or bulk. There is a place for both and you can mix up some of your routines to train both types of muscle fiber. Repetitions that are slow should be somewhere around a second or two and slow should be three to four seconds. Working one a few weeks and then another could be helpful here.