The answer to 'supply line slow draining' is that a slow-draining supply line is often caused by a clog or buildup in the pipe. This can happen due to mineral deposits, debris, or other obstructions that restrict water flow. Symptoms may include a sink or tub that drains slowly or incompletely.
In many cases, you can try to clear the clog yourself using a plunger, drain snake, or chemical drain cleaner. However, if the clog is severe or you're unable to resolve the issue, it's best to call a professional plumber. They have the tools and expertise to thoroughly inspect the supply line, identify the root cause, and provide a long-term solution.