One outdoor faucet water pressure has decreased dramatically in the last month. Other two are fine. No new plumbing installations recently. Copper pipes. No leaks detected. Easy to access. Silcock looks to be 10 yrs old or less. House built in 1967. Sound like a new spigot or just a washer needed??

Turn the water off at the house stop. Now unscrew the bonnet packing nut, (see image) and back the stem and washer assembly out counterclockwise.
Turn the water back on for a second to see if the supply's clear. There should be a full stream. Check the valve body to be sure it's clear and that the washer's intact and not worn out o9r cracked. Reassemble and see if it doesn't work better now. Regards, Tom