When it comes to HIIT speed and rep count are important because they help maintain intensity and it's intensity that delivers results. So, really, for Huff & Puff as for all of the High Intensity Interval Training workouts you want to start as fast and hard as you can, improve on it after the first set or two and then maintain the intensity by keeping track of the rep count for each exercise. That way you're really pushing against the edges of your ability and forcing your body to improve.