A slow shower and a trickling kitchen tap feel like one problem, but the cause can be your treatment equipment, the water supply coming into the house, or something simple like a clogged faucet aerator. Treatment is the most expensive guess, so start by finding where the drop begins.
Start with the faucets, not the equipment
Run each fixture one at a time and note what you see. Check the cold kitchen tap, the cold bathroom tap, the hot tap, and an outdoor hose spigot if you have one. If only one fixture is weak and the rest are normal, the problem is almost certainly local. Unscrew the aerator at the tip of that faucet and look for grit, scale, or a mesh caked with debris. Clean it, reinstall it, and test again. If hot water is weak but cold is normal, look at the water heater and its inlet valve before you blame treatment, because sediment inside a tank can narrow flow to the hot side only.
If every cold fixture is weak at the same time, the restriction is upstream of all of them. Treatment equipment is one candidate, but so is the supply itself.
Decide whether the drop is in the supply
Ask neighbors on your street whether they notice the same thing. A main break, a utility flushing program, or work on a shared line can lower pressure for everyone at once, and utilities usually post service notices when they do. Check the main shutoff and the valve at the meter as well. A valve left partly closed after a repair or a plumbing job is an easy miss, and confirming it is fully open takes a minute.
If your home is on a well, the pump, the pressure switch, and the storage tank are the first places to look. A pump that runs more often than it used to, or that short-cycles, points to a well-side problem that a water treatment provider is not the right person to fix. Call a well contractor for that.
Check the equipment only after the supply is cleared
If the house has a whole-house filter or a softener with gauges, compare the reading on the inlet side with the reading on the outlet side while water is running. A large pressure difference across the unit points to restriction inside the housing, such as a loaded sediment element. A normal inlet reading with a low outlet reading also points to the equipment. If there are no gauges, note whether the drop started after a filter change or after a softener cycle that may not have finished. Replacement intervals for filter elements are printed on the housing or in the manual, so use those rather than guessing from how the water looks.
Questions to ask before you pay for a visit
Ask whether the technician will measure pressure at the inlet and outlet of the system and write the numbers down for you. Ask whether they will tell you if the cause sits outside the system, and whether they will point you to the right trade for that. Ask what the visit costs if the cause turns out to be a supply valve, a well component, or a clogged aerator, and get that answer before anyone arrives. A provider who explains the diagnosis clearly, even when the answer is 'it is not us,' is worth more than one who sells equipment on the first call.
What to write down
Keep a short note with four facts: which taps are weak, whether the problem is hot, cold, or both, what time of day it happens, and whether neighbors are affected. When you can also say when the drop started, you have usually separated a local clog from a supply problem from an equipment restriction, and any service call will be shorter and cheaper for it.
If you want a provider who reads gauges and explains a diagnosis before selling anything, the directory of water treatment providers in Missouri is a place to start, and our methodology page explains how those providers are reviewed.