Lots of variables affect the rate that your RO/DI unit will process water. Water temp, water pressure, and TDS of the tap water being the primary ones. Tap water TDS (total dissolved solids) doesn't really affect output so much as membrane life. If you don't backflush though, you will notice a drop in flow as the membrane clogs. If you have low water pressure, it's hard to get decent output from an RO membrane because there isn't enough pressure to force the water molecules through the membrane and you get a high rejection rate...lots of waste water. If your water pressure at the sediment filter is less than 50psi, you might consider a booster pump. Also, you might want to consider installing pressure gauges before and after the sediment/carbon filters. A noticeable pressure drop across those filters indicates that they need replacement. Here area couple of pics that may help illustrate what I talking about. The first is an RO unit with a double DI. Note the pressure gauges and the TDS meter...these tell you how your unit is performing. Second is a chloramine filter used for top off on a FW tank...pressure gauges are easier to see here. JMHO.