Each prop tip has a trailing vortex just like a wing tip. That trailing vortex produces a substantial portion of the total drag on the prop when the prop is thrusting hard.

The duct of the ducted fan cuts the tip vortex at each blade tip which drastically reduces the induced drag and allows more of the engine power to be used to accelerate the flow through the duct. A well designed duct actually increases the efficiency of the system compared to an unducted setup.