Efficiency of RO membranes is temperature dependent. Cold water will decrease efficiency. They are rated at 25°C (77°F), so a 50gpd membrane may only produce 30gal if the temp of your cold water line is only 68°F. If you can take the water supply line and coil it up inside of a 5 gallon bucket filled with water and an aquarium heater, you will increase the temp of the water going through the membrane helping to increase efficiency.
A booster pump will also help if your water pressure is low but they cost about $200.
Since you have pretty hard water, don't forget to flush your membrane at least once a month. If you don't have a flush kit installed, you can do it manually too. Just remove the restrictor from the waste line and let it run for about 10 minutes.
