This thesis researches the performance and SEO implications of Server-Side Ren-dering (SSR) and Client-Side Rendering (CSR) in content-heavy web applications. Using the Next.js 15 framework, two identical applications, one rendered via SSRand the other via CSR, were developed and tested across key performance metrics, Time to First Byte (TTFB), Largest Contentful Paint (LCP), and Interaction toNext Paint (INP). Tests were conducted under both fast (WiFi) and throttling (3G)network conditions. SEO performance was evaluated using both automated tools,Lighthouse and Screaming Frog, and manual inspection. Results showed that while CSR achieved slightly better average LCP under ideal con-ditions, it exhibited high variability and severe performance degradation on slowernetworks. SSR demonstrated more consistent performance and significantly better LCP under throttled conditions. In terms of SEO, both approaches achieved perfect Lighthouse scores, but only SSR pre-rendered full content and media assets in HTML,enabling more complete indexing by search engines. CSR’s reliance on Java Script delayed content visibility to crawlers, potentially hindering SEO.The study concludes that SSR is more suitable for content-heavy applications where consistent performance and robust SEO are priorities, whereas CSR is more viablein controlled, app-like environments with stable network conditions.