It is a myth, though one clearly believed by some New Zealanders, that the United States economy has grown faster than the New Zealand economy over the last decade because the US central bank has a mandate to encourage growth and maintain price stability, whereas the New Zealand central bank has only a mandate to maintain price stability.
The reality is that in both countries it is now fully recognised that the best thing which monetary policy can do to encourage growth is to keep inflation under control. In that respect, the Federal Reserve and the Reserve Bank of New Zealand think and operate in the same way.