Developers can ramp up more quickly on new APIs, providing quicker feedback to the platform while the APIs are still the most malleable. Mistakes in APIs can be corrected quickly by the developers who use them, and library authors who serve them, providing high-fidelity, critical feedback to browser vendors and platform designers.

Why does test suite performance matter? First of all, testing is a part of a developer's feedback loop (see @searls talk) and, secondly, it is a part of a deployment cycle.

It is hard to appreciate how important really fast (and reliable) hot reload can be during development, unless you have experienced it yourself. Developers report that it changes the way they create their apps, describing it as being like painting their app to life.

But even that “minute” affects my feedback loop (see Justin Searls talk) and wastes my time.