John
V = L x di/dt where L and di are constants. The smaller V is, the longer dt is, that is the time for the current in the coil to dissipate. With a diode, V is 0.7 volts. Without a diode, V is whatever the break-over voltage of the contact is.
"When used with a DC coil relay, a flyback diode can cause delayed drop-out of the contacts when power is removed, due to the continued circulation of current in the relay coil and diode."
Above quote from below link.
https://en.wikipedia.org/wiki/Flyback_diode