Cooling time scales roughly with the square of wall thickness: double the thickness and cooling takes about four times as long. That is why a single thick area can set the cycle for the whole part.
Three signs that mold cooling isn't working: the temperature difference between inlet and outlet water of a circuit is more than a few degrees, the flow is laminar instead of turbulent, or the channels are scaled. Even a few tenths of a millimetre of scale acts like insulation.
On many lines the cooling time was once set long "to be safe" and never revisited. By measuring part temperature at ejection and monitoring dimensions, a few seconds can usually be removed without any loss of quality.
This article is general guidance. The figures depend on each plant's conditions; for decisions, measurements on your own line are what count.