Compare pressure with the softener in normal service versus bypass. If pressure returns in bypass, the restriction is inside the softener or its valve/plumbing.
Fouled or swollen resin
Iron, sediment and other contaminants can foul resin and increase pressure drop. Resin can also degrade with age or chlorine exposure.
Control valve restriction
A partially blocked valve, distributor or internal screen can restrict flow even when the resin itself is usable.
System may be undersized
A softener with adequate grain capacity can still have insufficient service flow for multiple showers and appliances at once.
Replacement opportunities
RKIN, SpringWell and Kind Water publish service-flow ratings that make them good comparison targets when an old softener is clearly undersized.
Sizing and chemistry checks for Low Water Pressure After a Water Softener: Resin, Valve or Sizing?
Water-softener decisions should be based on measured hardness, expected daily water use and peak service flow. “Grain capacity” is useful only when interpreted with the programmed salt dose and regeneration efficiency; the largest number on the product page is not automatically the most efficient operating point.
Iron changes the sizing problem. Some dissolved iron can be handled by ion exchange, but meaningful iron loading can foul resin and increase maintenance. When testing shows significant iron, manganese or sulfur, dedicated pretreatment is often more robust than asking the softener to solve every well-water problem.
Compare regeneration efficiency and long-term ownership
A well-sized unit should regenerate often enough to protect resin capacity without wasting salt and water. Compare usable capacity at the intended salt setting, estimated gallons between regenerations, service-flow rating and the amount of water used during regeneration. These operating details matter more over ten years than a small difference in purchase price.
Also consider valve support, replacement resin availability, bypass plumbing and local drain requirements. Salt-free conditioners belong in a separate category: they may reduce scale behavior in appropriate water but do not remove calcium and magnesium the way a true ion-exchange softener does.