The primary challenge in scaling a web application is managing performance while ensuring consistent user experience under increased loads. As user traffic grows, issues such as latency, server capacity, and data management become critical. Balancing resource allocation while optimizing the application's architecture is essential to prevent downtime and sluggishness. Developers must also consider the impact of scaling on security and data integrity. Strategies such as load balancing, database partitioning, and utilizing cloud infrastructure can help address these challenges, enabling applications to scale efficiently without compromising functionality or security.