tl;dr
The correct answer to what burns slower, a "lean" or "rich" mix is lean, not "expansion pipe".
Many things affect the air to fuel ratio; jetting, weather, expansion pipes, even the amount of oil in the gas. Regardless of these contributing factors, you can only have a "lean" or a "rich" mix. A slightly lean mixtures burns slower, hotter and more efficiently. A slightly rich mixture burns faster, cooler and with more power.
And that's all there is to it. :)