In my experience, the primary causes of poor performance in Next.js applications are the following:
1. Inefficient data fetching and processing
1.1.
Sequential requests (fetch waterfalls) instead of parallel ones.
1.2.
Slow queries to the database/API.
1.3.
Over-fetching.
1.4.
Blocking operations on the server that affect Time to First Byte (TTFB).
2. Suboptimal rendering and caching strategy
2.1.
Using dynamic rendering (rendering at request time) for content suitable for static rendering (rendering at build time or upon revalidation).
2.2. Inefficient use of server-side caching mechanisms in the App Router
2.2.1.
Incorrect data cache configuration.
This includes unnecessarily using the cache: 'no-store' option for fetch requests or failing to utilize unstable_cache for data that could be cached.
This increases the load on data sources.
2.2.2.
Suboptimal revalidation strategies for data cache and full route cache (time-based revalidation or on-demand revalidation), leading to the display of stale data or too frequent content regeneration.
2.2.3.
Unintentional disabling of the full route cache due to the use of dynamic functions (e.g., cookies(), headers()), opting the route into dynamic rendering and preventing caching of the rendering result.
2.2.4.
Failing to use React cache for non-fetch requests (e.g., via an ORM or direct database queries), resulting in the duplication of identical requests within a single render pass, as these requests (unlike fetch requests) are not automatically memoized.
3. Large JavaScript bundles and hydration issues
3.1.
Importing unnecessary libraries.
3.2.
Insufficiently granular component-level code splitting: failing to utilize lazy loading (with next/dynamic or React.lazy) for non-critical or large client components.
3.3.
Slow hydration due to excessive size or complexity of client components, leading to increased Time to Interactive (TTI) and Total Blocking Time (TBT).
3.4.
Using client components instead of server components for non-interactive parts of the interface, leading to an excessive JavaScript bundle size and increased hydration workload.
4. Unoptimized resources and third-party scripts
4.1.
Failing to use next/image for automatic image optimization (e.g., serving modern formats like WebP/AVIF, responsive sizing, and lazy loading).
4.2.
Render-blocking fonts and third-party scripts (analytics, ads).
5 Infrastructure configuration issues (Cloudflare)
5.1.
Inefficient CDN caching rules.
5.2.
Runtime environment limitations (e.g., Edge Runtime), such as resource constraints (CPU time, bundle size) and incompatibilities with third-party libraries that rely on unsupported Node.js APIs.