This thesis details the strategic modernization of Höganäs AB's legacy web applications, transitioning from a VB-based architecture to a modern .NET 8 and C# framework with Blazor integration. The primary objectives were to enhance performance, scalability, and maintainability while preserving the core functionality. The modernization involved a comprehensive code migration, adoption of modular design principles, and Blazor integration.

Performance testing with JMeter revealed significant improvements, with the new C# application reducing average response time from 578 ms to 39 ms with 10 simulated users. The C# application also demonstrated greater stability under stress, with lower standard deviations in response times.

The findings confirm the effectiveness of the modernization strategy, showcasing enhanced system performance and scalability. This thesis offers a practical framework for organizations undertaking similar legacy system modernization projects.