There are exceptions, and more and more strong guys including NFL teams are now using slow-speed training. You do not have to train 10 up and 5 down. I m a regular-speed guy, but still use slow training as a change of pace. At least once a month we will use it. The speed of motion I ve been using for this is 8 seconds up and 8 seconds down, for 8 reps to add weight.
Slow training is especially good for rehab.
Don t let the so-called experts with narrow definitions of science confuse you. Remember the basic hardgainer philosophy of hard work, good food, and plenty of recovery time. Combine this with lots of effort and dedication, and you can succeed with one of many specific modes or methods of training.