That's entirely right; to explain a bit more, the reason that index is good is because it's a <i>covering</i> index.  You have all the data you need to pull from the table in the index (which you've found via binary search through your B+tree), so you don't need to run out again to the table, and that saves an extra disk seek.<p>To make a covering index takes longer, so insertion speed drops, but that shouldn't be the hotspot.