But to finish, tapering must start—for a given pace of tapering, the longer it takes to start tapering, the longer it will be before the policy rate can be raised. Of course, one can keep the liftoff date fixed and simply taper at a much faster rate, including the possibility of a hard stop of asset purchases.
But concerns about financial market functioning, including the ability of markets to absorb the purchases the Fed stops making, typically limits how fast the tapering can be, particularly given the amount of asset purchases we were making at the time $ billion per month).