If you're in a hurry for ice cubes, maybe you are having a party, leave 3 or 4 frozen cubes in the tray and then fill the other partitions with cold water. The new ice cubes will freeze in half the time.
If you use water that has been boiled first and allowed to cool. That too will speed up the freezing process. Now why this is so I don't know, but it is worth a try if you have a stop watch and a few hours to spare.