You don’t need to be a king air pilot to answer that.
You never feather a propeller to slow down. Increasing prop speed on any airplane adds drag, basically it creates a larger surface area against the relative wind. (a large flat disk has more drag than 2, or 3, or 4 paddles.)
That’s the extreme, talking about actually feathering.
Slowing the prop speed works more or less the same way. Turboprops are different than pistons with constant speed props. Don’t forget the turbine is spinning at something like 30,000 rpm, while the prop is spinning somewhere between 1600 and 2000 rpm. In a piston the prop speed equals the engine speed, so it’s hard to compare the two when talking about the principles behind the operation.
Think of it like engine braking in a vehicle. In a low gear, with the engine idling (high prop speed, low torque) slows you down a lot faster than putting the vehicle in neutral (slow prop speed or feathering).