I have not used this device and I'm not 100% sure but...
The device uses a low side current sense resistor. This means that during the off period, if you use coast/all switches off, the current thru the sense resistor reverses which produces a negative voltage on the sense input, an event from which the sense amplifier in the chip then needs to recover - and this happens every PWM cycle.
When using slow decay/both low side switches on, the current never flows thru the sense resistor during the off period of the normal PWM cycle, hence no negative voltage on the sense pin. Do note, however, that the device itself uses coast/all switches off during the current regulation off time.
Hopefully (surely) someone will correct me if I'm wrong :-)