The SlewRate is a scale on how quickly the smoothed changes to DelayScale will occur (when SmoothDelayChanges is checked). The greater the value of SlewRate, the faster the change in delay time. Changes to DelayScale are functionally equivalent to a Doppler shift in the air and, like the Doppler effect, can result in a "whooshing" sound or a transient pitch shift (down for lengthening delays, or up for shortening delays).
Larger values of the SlewRate will cause the delay through the DelayWithFeedback to follow changes to the DelayScale parameter more quickly. If SlewRate is set to 1, the time it takes for the delay time to "catch up" to the DelayScale is the same as the time difference (that is, if the delay is currently 5 seconds and you set the DelayScale to ask for 10 seconds delay, it will take 5 seconds for the DelayWithFeedback to transition from a 5 second delay to a 10 second delay).