Regular/Maximum Fan Speed Threshold configures a threshold of the layer printing time to determine the fan speed used in the layer.
If the printing time of a layer is equal to or larger than the Regular/Maximum Fan Speed Threshold, then this layer will be printed with the Regular Fan Speed.
Fan Speed of the Layer = Maximum Fan Speed - (Maximum Fan Speed - Regular Fan Speed)/(Regular/Maximum Fan Speed Threshold - Minimum Layer Time) × (Printing Time of the Layer - Minimum Layer Time)
If a layer is printed with the Minimum Layer Time, then this layer will be printed with the Maximum Fan Speed.
If the printing time of a layer is short, the layer may not be able to cool down properly. That's why we need to increase the fan speed to ensure that the layer is solidified.