Start with whether the problem affects every fixture. Then compare pressure/flow with the treatment in normal service versus manufacturer-approved bypass to isolate the restriction.
Most common cause: loaded cartridge or prefilter
Sediment and carbon cartridges accumulate material and become more restrictive over time. A sudden improvement after replacement strongly points to loading.
System may be undersized for peak demand
A filter rated for modest flow can feel fine at one faucet yet starve the house when multiple showers and appliances run simultaneously.
Compare service flow, not just peak marketing flow.
Too-fine filtration can create unnecessary restriction
Using a very fine sediment cartridge upstream when the water does not require it can create avoidable pressure loss and short filter life.
When a booster pump is appropriate
A booster pump can help when incoming pressure is genuinely inadequate, but it should not be used to mask a clogged or undersized treatment system. Diagnose the restriction first.