Quick take
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.