Low water pressure

Does it have an on-board filter?

Disconnect a hot or cold hose to faucet....briefly turn on water supply with hose in a bucket to check flow/pressure

If Good...replace faucet cartridge or faucet

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IS the kitchen faucet the kind where you grab the arm and pull it out and use it as a sprayer. Or the kind with a sprayer off to the side.

In that order
At teh "Base" of the pull out part (Where the hose connects) is some kind of backflow preventer. THis may have "Gunked up" and needs cleaning.. A vinegar boil may do it. I don't really like the chemical cleaners. or get a new back flow preventer.

UNSCREW teh hose and it should be right there.. You may be able to tap-tap-tap it out.

On the 2nd
IN the faucet base (You will need to remove the faucet arm is a "DIVERTER" valve. Same thing.

I must be something after the mixing valve if it was something before both the hot and cold would have to have the same problem.

Hi,

Try cleaning the aerator.

As mentioned above, unscrew the hot and cold PEX line fittings to the faucet under the sink and see if there's flow. Also check that the flow rate is about the same. If good flow in hot & cold, would point to the faucet being the issue.

If you remove the aerator, do you get good flow? If yes, then clean the aerator. If no, the mixing valve could be suspect. Could be plugged up or defective - clean/replace as necessary. If a drop down faucet, can disconnect the flex hose under the sink to see if you get any flow when faucet turned on (use a bucket or pot to collect water). If yes, a plugged or defective drop-down hose - try cleaning or replace as needed.

The faucet have a screen be sure is clean sometime it get stuck with debris from the water heater.(calcium)

I had exactly the same problem. My faucet was a Phoenix brand with a separate sink sprayer. The diverter to the sink sprayer was stuck. No parts were sold for this faucet, so I went to Lowes and purchased a standard name brand replacement faucet. That solved the problem.