Power BI consultants help with performance optimization by analyzing and refining data models, DAX queries, and report designs to ensure they run efficiently. They implement best practices such as minimizing the use of complex calculations, optimizing data relationships, and using aggregations and indexing. Power BI developers also assess the infrastructure, ensuring that Power BI is correctly configured to handle large datasets and high user loads. These optimizations lead to faster report loading times, more responsive dashboards, and an overall better user experience.