Provide a comprehensive explanation of database read replicas. Your explanation should cover the following key aspects in detail:

1. Concept of a Read Replica: Define what a read replica is and the primary problem it aims to solve (e.g., high read traffic on a primary database).
2. How it Works: Describe the fundamental mechanism of how a read replica functions, including the process of data synchronization (e.g., asynchronous replication, log shipping) from the primary database to the replica.
3. Replication Lag: Explain what 'replication lag' is, its common causes (e.g., network latency, replica workload, primary write spikes), and its potential implications for application data consistency.
4. Scaling Read Performance: Detail practical strategies for leveraging read replicas to scale the read performance of an application. Discuss concepts like read/write splitting, load balancing across multiple replicas, and considerations for application architecture changes.