Size the filter so normal household demand stays below its continuous service-flow rating, and monitor pressure before and after the system. A clean system that causes unacceptable shower pressure is likely undersized or installed in an overly restrictive plumbing path.
Pressure drop rises with flow
As household GPM increases, restriction through valves, media and cartridges becomes more noticeable.
Dirty prefilters are common culprits
A sediment cartridge can create much more pressure loss near the end of its life than when new.
Multiple treatment stages stack resistance
Carbon, softener, iron filter, UV plumbing and cartridge housings all add some resistance. Size the complete train, not each component in isolation.
Use service flow as a guardrail
Staying below the manufacturer’s continuous service-flow rating is a practical starting point for avoiding unnecessary pressure complaints.