Thermodynamics 101. There is no such thing as cold, its always heat-the absence of heat is why you feel cold. Coolers works by removing heat thats why they are also called heat exchangers. Any object with volume and density is subject to heat. An empty ref will be easier to remove heat than a loaded one. you fill in the ref with water jugs then you are just adding up heat, compressor needs more work to maintain the thermostat. If you really want to speed up cooling then remove air inside the ref, unfortunately theres no such vacuum-ref in the market you need to customize in the lab.