The answer to 'supply line slow draining' is that a slow-draining supply line is typically caused by a clogged or partially blocked pipe. This can happen due to buildup of mineral deposits, hair, or other debris over time. Symptoms may include water draining slowly from sinks, tubs, or showers.
In many cases, this issue can be resolved with basic DIY troubleshooting and cleaning. However, if the problem persists or the clog is severe, it's best to call a professional plumber to inspect the line and use specialized tools to clear the blockage.