reduce cooling time of cast aluminium

in what way are you allowing the cast to cool ?

if you are allowing it to cool by leaving it just like that then you can circulate coolants around the die/mould . that may increase the rate of heat transfer and reduce the cooling time.

If the cooling time in any way affects the physical properties, then you will have to determine by trial and error, experimentally a lesser cooling time which may not affect the properties and like wise you may have to design your cooling system.

More data needed to give sensible response. Are you talking of cooling after demoulding? If the alloy has silicon, the grain structure will be modified by the cooling process. What sort of properties do you want unchanged?
Aluminium being a good conductor of heat the cooling rate is determined by the surface heat transfer rather than internal conducted transfer. Blowing air over the component will moderately increase the cooling rate. Sprinkling water will be faster. Immersion in agitated water will be the fastest.