Yes, it does slow down the turn off of the coil. Usually a zener diode is used - not a regular diode - to improve the turn off. You need the diode to prevent the voltage from the inductor from rising to the point it breaks down the switching transistor. You can select the zener(s) for a voltage safely below the Vceo of the transistor to optimize it, but remember the zener is dissipating the energy stored in the coil so needs to have a high enough power rating.