What causes slow website performance?

Common causes include unoptimized images (too large, wrong format), excessive JavaScript (large bundles, render-blocking scripts), unoptimized CSS (large files, render-blocking), slow server response times, lack of caching, too many HTTP requests, large page sizes, third-party scripts (analytics, ads, widgets), inefficient database queries, and poor hosting. Mobile-specific issues include lack of mobile optimization and heavy resources. Identifying the specific cause requires performance monitoring tools. Example: A site's slow performance was caused by a combination of unoptimized images (40% of page weight), large JavaScript bundles, and slow server response times—addressing all three improved performance significantly.