Why every trainer, bodybuilding expert, book, etc. says we have to perform very slow movements and when you watch pro - like Arnold, R. Coleman, J. Cutler & Co. - they "pump" and also very fast?!? The only one seems to be Mentzer...
slow movements emphasize proper form. If you go too fast when you're starting out, you'll loose form and technique and you wont get as much out of your workout. Work out slow until your body does that form in exhaustion, then you can speed things up