Actually it doesn't burn slower, just takes a higher temp to ignite. Well maybe the flame front accelerates a bit slower at first as it is harder to ignite, but ultimately it catches up. The flame front has about the same speed. Diesel fuel really does burn slower but it wound detonate like crazy. But it takes a much lower temperature to ignite. This makes it great for a Diesel engine as it doesn't need as much compression to ignite. But becaue it burns slower the engine will not rev as high. Diesel also has a higher BTU content, that along with high compression ratios contribute to a Diesel engines high torque.