by Why does my kitchen sink drain slowly even after cleaning the trap?
Short Answer: The restriction may be farther downstream than the sink trap.
Detailed Answer: If trap cleaning does not improve drainage, wastewater may be encountering resistance in the branch line, main sewer line, septic tank outlet filter, pump system, or drain field. Multiple slow fixtures increase the likelihood of a septic-related problem.
Related Questions: How do I know if a septic tank outlet filter is clogged? Why are several drains slow at once? Can septic failure affect kitchen sinks?