What little I know: "Head speed change rate" controls how fast the ESC will change the speed -- if it gets a faster change in commanded speed, it'll ramp the speed toward the new target at the change rate until it's close. This is to prevent a powerful motor from stressing a helicopter's gear train.
It doesn't matter if you're using a Hubin (or Hubinesque, if anyone is copying Will) timer, but it matters with the Igor timer and (I think) the Fioretti (sp?) timer.