The answer to 'why is my shut-off valve low water pressure' is that a partially closed or faulty shut-off valve can restrict the flow of water and reduce water pressure. This is a common issue that can occur over time as the valve mechanism wears down or gets stuck.
Some common symptoms of a shut-off valve causing low water pressure include reduced flow from faucets and appliances, difficulty turning the valve, and inconsistent water pressure. To troubleshoot, you can try fully opening and closing the valve a few times to see if that improves the pressure. If the issue persists, the valve may need to be repaired or replaced by a professional plumber.