This deceleration of growth can be a result from population ageing (which would increase the saving rate), or a general deceleration of productivity growth (reducing investment), or both. Now, if the natural rate has indeed declined, monetary policy in the course of normalisation becomes tighter at an earlier stage than the past experience of historical interest rates would suggest.
A given rise in policy rates would result in a higher degree of monetary tightening than a backward-looking analyst or policy maker would expect.