Speed is the whole argument
Half your visitors leave before a slow page finishes drawing. Here is what actually moves the number, and what only looks like it does.
Why the first second decides it
Most visitors leave before a slow page finishes drawing. Not because they judged it, but because they never saw it. A sub-second first paint is not a vanity number.
- Images sized before they load
- Fonts that do not block the render
Speed is a feature you feel before you can name it.
More in the Web2Sales writeup.