The proliferation of dynamic and interactive web applications necessitates robust Web Performance Optimization (WPO) to meet user expectations for speed, responsiveness, and reliability. While numerous WPO techniques exist, understanding their practical impact on both objective metrics and perceived user experience (UX), alongside the challenges faced by developers in implementing them, remain crucial. This thesis investigates the effectiveness of various frontend WPO techniques, explores the hurdles encountered in development practice when implementing these client-side optimizations, and examines strategies for achieving sustainable optimization while maintaining accessibility and user satisfaction. Employing a mixed-methods approach, this study combines empirical testing using a controlled web application (comparing unoptimized, moderately optimized, and fully optimized versions) with qualitative user feedback and a survey targeting web developers. Key findings reveal that caching strategies, comprehensive image optimization, efficient asset loading techniques (including code splitting, lazy loading, and minification), and optimized rendering paths yield the most significant performance improvements, correlating strongly with enhanced user perception. However, implementation is frequently hindered by organizational challenges such as time constraints due to feature prioritization, a lack of specialized expertise, difficulties in measuring impact, and technical limitations associated with legacy systems. The research concludes that achieving effective and sustainable WPO requires a holistic approach, integrating performance considerations continuously throughout the development lifecycle, balancing technical gains with accessibility needs, and fostering an organizational culture that prioritizes performance alongside feature development. This work contributes evidence-based insights bridging technical frontend WPO effectiveness, perceived UX, and practical implementation realities, offering guidance for developers and organizations aiming to build faster, more efficient, and user-centric web applications through targeted client-side optimizations.