RTO stands for Recovery Time Objective in the context of disaster recovery. It represents the targeted duration within which a system or service must be restored after a disruption or failure. Achieving a low RTO is essential for minimizing downtime and ensuring business continuity during disaster recovery scenarios. 

Denormalization involves combining normalized tables to reduce redundancy and improve query performance by eliminating the need for excessive joins. This often leads to faster query execution. 

In addressing slow query performance, normalization is essential to minimize redundancy and improve data organization, thus enhancing query performance. However, denormalization can also be beneficial in specific cases, such as eliminating joins to reduce query execution time. Partitioning large tables and indexes can enhance parallel processing and improve query performance. Additionally, optimizing SQL queries and utilizing proper indexing techniques are crucial for optimizing database performance. 

In Visual Explain, a red arrow between nodes typically indicates data movement. This means that data is being transferred between different stages of query processing, such as between tables or between sorting operations. Understanding data movement can help optimize query performance by minimizing unnecessary data transfers. 

The buffer pool in DB2's architecture plays a crucial role in caching data and managing data in memory. It stores frequently accessed data pages in memory, reducing the need for disk I/O operations. This improves performance by speeding up data retrieval and manipulation operations. 

To determine if an index is being used by the query optimizer in DB2, you can analyze the explain plan of the query. The explain plan shows the execution steps chosen by the optimizer, including whether indexes are utilized. 

DEFERRABLE constraint in DB2 postpones the constraint checking until the next COMMIT or ROLLBACK operation, allowing flexibility in handling constraints within a transaction. This feature is particularly useful in scenarios where you need to temporarily violate constraints during the transaction and enforce them only at the end. 

Performing regular audits and compliance checks helps in identifying deviations from established standards and ensures adherence to regulatory requirements. It allows the administrator to take corrective actions, such as updating security policies or access controls, to mitigate risks and maintain compliance. 

Buffer Pool is often optimized during performance tuning in DB2. It's a memory area where data pages are read from and written to disk, and optimizing its size and configuration can significantly impact performance. 

The UPDATE command in SQL is used to modify existing records in a table. It allows users to change the values of one or more columns in one or more rows in a table based on specified conditions. It's commonly used to update data when there are changes or corrections needed in the database.