The problem, I feel, most coaches have with giving the proper rest period to develop speed is they don't feel they can provide that much time. It's all about planning. If you know the athletes will need three minutes to rest between sprints, plan non-compete exercises like balance exercises, partner toss, and catch, quick hand drills, etc., during the rest. This strategy will help develop an ability/skill while resting- and gives the athletes more buy-in. Plan ahead of time for a proper rest.