by Why does my shower drain slowly only during heavy water use?
Short Answer: The system may be operating near its maximum capacity.
Detailed Answer: Light usage may drain normally, but simultaneous showers, laundry, dishwashing, and toilet flushing can overwhelm a partially restricted sewer or septic system, causing the shower to drain slowly during peak demand.
Related Questions: Can septic problems appear only during busy periods? What causes intermittent slow drains? How much water can a septic system handle?