Mechanical brakes always operate below 5 mph, otherwise this brake noise when creeping wouldn't exist. I only suffer from the issue coming to a complete stop and then slowly releasing the brakes (creeping). Driving slowly around a car park doesn't seem to be an issue.
As previously mentioned in this thread, high speed heavy braking will tend to clean up the pad and disc surfaces to stop the noise from occurring, at least for a short time. It seems to be caused by glazing of the pad surface from the mechanical brakes only providing very light assistance during higher speed braking, with the regen braking doing most of the work. The lighter the driver is on the brakes the worse it seems to be. Not sure why the IS seems to suffer more than other hybrids, my old RX400h or my wife's old Prius Gen 2 and current Gen 4 don't suffer from this.