Truthfully, bringing the bar down fast won't mean anything if you aren't tight and in your groove. For that reason I would say that beginners should probably not worry the down speed as much as blowing it off their chest as fast as possible. I'm not saying a 3 second trip from lockout to your chest, but don't try to just whiz the bar all over. Down as fast as you can while staying in your groove and under control. Bar should touch in the same spot each and every touch.