A small case for building slowly
Speed is useful. Knowing what to keep is harder.
Sample writing
It has become remarkably easy to make a first version of something. A sketch can become a working page in an afternoon. A vague idea can become a prototype before you have found the right words to describe it.
That is a good thing. Making an idea tangible is one of the fastest ways to learn what you actually mean.
But a first version answers a particular question: can this exist? It says much less about whether it should stay, or what it will feel like to use repeatedly.
Give the idea an ordinary day
A prototype deserves time outside the excitement of making it. Use it for an ordinary task. Come back when you are tired. Try to remember how it works without reading your own notes.
The parts you were proudest of may turn out to be the parts you keep working around. An unremarkable little feature may become the reason you open the tool at all.
This kind of information arrives through use. It cannot always be accelerated by writing more code.
Slowness as a deliberate choice
Building slowly does not mean delaying everything. It means choosing where a little waiting is valuable.
Move quickly when testing an assumption. Take more care when setting a convention that every future feature will inherit. Make the experiment disposable; make the foundations understandable.
A useful prototype makes the next question clearer.
Some days the next step is another feature. Other days it is deleting one, improving a sentence, or watching someone try to do something that seemed obvious to you.
The point is to leave enough space between making and deciding. That space is where a working thing starts to become a worthwhile one.