It's not that the stem is attached more "firmly", it's that a 1 1/8 steerer tube clamp diameter is much larger than a quill stem. Along with the fact most modern bars have a larger diameter. It also depends on how high you have a quill stem exposed from the steerer. But comparing my modern and vintage bikes by physically seeing how much I can move the bars, the difference isn't all that much. It certainly won't make a difference in anyone's speed. And another way to look at it if a quill setup flexes more it theoretically will ride smoother on rough roads. That's my idea, anyway.