Reducing Emissions is the highest priority for Prius.  It strives to remain a SULEV (Super Ultra Low Emission Vehicle) at all times,  which means the catalytic-converter must be kept warm even if that requires consuming some gas to accomplish that.  Fortunately, you still get better than average mileage, even if the engine doesn't shut off right away. Creating heat for the emissions system and heat to keep you warm is fastest and most efficient if you simply drive gently immediately after starting the Prius in the winter.  Allowing the engine to run while the Prius is parked takes longer and is less efficient.  "Just Drive It!. The user-manual states to turn the key and hold it until you hear a beep.  Realistically, you can actually let go of the key immediately.  Once the computer accepts your request, the engine will start-up after preparation is complete.  Think of it as clicking a button on a computer-mouse.  The catch is you must still wait for the "READY" prompt before shifting into Reverse or Drive. All the information stated in this document was provided by Prius owners.  None were affiliated with Toyota Motor Corporation, except as customers. This document is not sanctioned by Toyota Motor Corporation or any of its affiliates