This a YouTube video by FitnessByMatt, interviewing Dr Layne Norton at the 2013 Arnold Classic. In the interview Matt asks Layne about the difference between fast and slow cardio. Just to clarify, fast cardio would be something like HIIT (high intensity interval training), and slow cardio would be something like a fast paced walk on an incline treadmill.
Layne goes on to explain the differences between both, and quotes a study where control groups had their body fat levels tested, and that the fast cardio group lost 50% more body fat despite working out for a much shorter amount of time. The slow cardio group trained for 1 hour, whereas the fast cardio group had 6 intervals of sprints lasting just 30 seconds each (3 minutes of work in total). He concedes that the actual workout time was likely 20-25 minutes for the fast cardio group as they were allowed as much rest as they wanted to recuperate in between sprints. Matt and Layne suggest that the increase in mitochondrial biogenesis plays a big part in the fat burning process post workout.
He also addresses that fact that many body builders believe that high intensity cardio is catabolic, where in fact, studies have shown that the short intensity bouts of cardio actually create an anabolic response. Interesting viewing.