When a bathroom or kitchen sink runs slowly even though no surface hairball or visible obstruction is found in the trap, the issue is usually caused by a partial restriction or narrowing of the pipe interior due to a lining of soap scum, grease, and mineral scale. While water can still force its way through the reduced opening, flow velocity is severely impaired. Alternatively, a blocked roof vent stack can restrict airflow and slow down drainage. Verification: Inspect the upper drain trap using a flashlight or clear out surface debris to test flow velocity.
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