How settled the scope is
The single biggest factor, and it is not close. A four-to-six-week MVP is not a faster build — it is a smaller question, and the window holds only because the scope is settled before anything is written. A brief that is a list of features rather than a question with a yes-or-no answer is a two-to-four-month build wearing a disguise.
What a four-to-six-week MVP includes