When your Node.js application experiences slow database queries, the issue could stem from various sources, including unoptimized indexes, inefficient query structures, or high data volume. The first step in troubleshooting is to use the database’s built-in profiling tools, such as MySQL’s `EXPLAIN` or MongoDB’s `explain()` function, to analyze how queries are being executed. These tools reveal whether the database is using the appropriate indexes and how much time is spent on each part of the query. Missing or poorly designed indexes are often the primary culprit for slow queries. Indexing frequently queried columns can drastically improve performance. Additionally, reviewing query structure to avoid costly operations like full table scans, reducing the amount of data retrieved, and caching frequently accessed data can provide significant performance boosts. Monitoring database performance over time and setting up alerts for slow queries can also help in identifying problems before they escalate.