When the outside bib is turn on, the water pressure to the entire house loses a considerable amount of pressure. Also the outside pressure from the outside bib also has low pressure. I recently had some piping(leak) replaced under the kitchen sink under the sub floor. The plumber also said the outside bib was leaking... he didn't have access to whee the leak was coming from, so added a new one and tied into the piping he replced below the sink... he tied right into it. Shortly after, we noticed low pressure when the outside water is turned on. It seems OK on the inside until you turn the outside water on. We are not using well water, and we do have a large water purification system installed in the laundry room that is hooked next to the main shut off valve. It has been there since 2006. Can you give me some suggestions? Thank you.

You need to get a pressure gauge and tell us the psi at the bib, water heater and sink. Was any fed pipe replaced possibly with a smaller line? Sounds more like volume than pressure.