The significant loss of flow through a softener, is commonly caused by damaged resin that requires replacement.
Many Municipal water suppliers have been adopting the use of Chloramine (chlorine + amonia) as a disinfectant, favoring it over plain chlorine. While constant chlorine exposure will damage any resin over time, damage from Chloramine will be more rapid.
If your water supplier is only using chlorine, although you stated the softener current contains 10% cross-linked resin, I suspect it is not 10%, and likely not even 8% CL (standard resin), particularly if the softener was obtained from an online dealer.
You may want to consider installing a backwashing carbon filtration system to remove the chlorine/chloramine prior to the softener. While common Granular Activated Carbon (GAC) will rapidly remove plain chlorine, Catalytic Carbon should be chosen to more effective to remove chloramines.
The recommended flow rate for either type of carbon to effectively remove chlorine, chloramine and other contaminants including the byproducts of disinfection is between 1 and 3 GPM per cubic foot of carbon media. A flow range is specified as some contaminants such as chloramines, are more stubborn to remove, thereby requiring longer contact time with the carbon to result in the most thorough removal, compared to less stubborn contaminants.
