On original galvanized pipe, the usual cause is the pipe itself: decades of internal rust narrow the line, so one fixture gets acceptable flow and two divide a straw. Fixtures and valves can cause local versions, but house wide fading on a 1950s Norwalk tract is a system reading until proven otherwise. The check is simple, a pressure and flow read at an outside bib with fixtures opened one by one. If the numbers describe narrowed pipe, no fixture repair fixes it, and the conversation becomes repair priorities or a repipe, with prices for both.
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