If you live in a very cold area, here’s a nice solution to keep your flow rate up even when your water supply is very cold:
Replace the tubing that leads from your cold water pipe to the unit with a 25' long piece. Coil as much of that tubing inside a 5 gallon bucket and fill the bucket up with water so the coils are submerged. Put an aquarium heater in the bucket and set it to 78° F. The heater will keep the bucket water at that temperature, and the water will warm up the water traveling through the tubing to your RO/DI unit. Production rates will jump back to normal.