Service pipefitter on deck. As stated above i would turn the hot water to the off position. Then close the supply line coming into the hot water heater. Open all the hot water taps on your faucets and back drain all the hot water lines by draining the hot water heater. If the sacrificial anode in the heater is completely gone the tank will rapidly start to corrode. Some faucets have screens at the outlet that can get clogged, but if its a shared spout and the cold runs fine that's probably not the issue. You can check the screens (if there is one)by turning a small ring at the outlet of the spout. Ive had calls where the screens are plugged and that was the only issue. you can check the sacrificial anode easily and get a generic one at any hardware store. Ide watch some you tube videos on the subject. If its a relatively new water heater its probably not the anode because they usually last ten years plus. If your lines go under ground and you cant see them to visually inspect them for leaks, kinks, or possible restrictions try turning everything off( supply to water heater, ice maker, etc) as stated above and see if the meter is still running. If it is indeed a leak the meter will continue to run. It would be a sizable leak to affect the pressure. If it were in the walls or floors you would see the damage pretty soon. You would definitely know its a leak if your water bill is unusually high. Check these things and post what you find.