Next.js 15: What's New and Migration Strategy for Enterprise Applications

In Next.js 14: fetch() requests were cached indefinitely by default, GET route handlers were cached, and server components were cached. In Next.js 15: fetch() is uncached by default, route handlers are dynamic by default, and only explicitly opted-in content is cached.

For one client's migration, this change increased database queries from 5,000/hour to 180,000/hour when we naively upgraded without adjusting caching strategies. Reverting to cached behavior requires explicit cache: 'force-cache' in fetch options and export const dynamic = 'force-static' for route handlers. Our recommendation: audit all data fetching during migration to explicitly define caching strategy.

Next.js 15 includes React 19 RC, which introduces breaking changes in the ecosystem. Major UI libraries have been slow to update: Framer Motion, react-hook-form, and many headless UI libraries had compatibility issues until recently. Before migrating, audit your dependencies using npx npm-check-updates -i and test in a staging environment.

For a client with 130+ dependencies, we discovered 18 packages requiring updates or temporary forks. Build time increased by 40% due to transpilation of incompatible libraries. Budget 20-30% more migration time than typical major version upgrades.

PPR enables mixing static and dynamic content in a single route without complex ISR configurations. A product page can show static product information immediately while streaming dynamic pricing and inventory data. For an e-commerce client, PPR reduced Time to First Byte from 450ms to 85ms while maintaining real-time inventory accuracy.

Implementation: wrap dynamic content in <Suspense> boundaries and enable experimental.ppr in next.config.js. One gotcha: PPR requires extensive edge function support from your hosting provider. Vercel fully supports it, but self-hosted solutions need careful edge infrastructure setup.

The new after() API enables fire-and-forget background tasks without blocking response. Use cases: analytics tracking, cache warming, notification sending. A SaaS platform reduced API response times by 150ms by moving analytics logging to after() calls, directly improving user-perceived performance.

Example: After form submission, immediately return success while sending confirmation emails and updating analytics in the background. This pattern works elegantly with serverless functions where you previously needed separate background job infrastructure.

Turbopack, the Rust-based bundler, is now the default for next dev. Our measurements show 76% faster cold start times and 40% faster hot module replacement compared to Webpack. For large applications (100,000+ lines), this translates to 5-8 seconds faster refresh during active development.

One caveat: Turbopack's plugin ecosystem is immature. If you rely on custom Webpack plugins (less common with modern Next.js), audit compatibility before switching.

Update all dependencies to Next.js 15-compatible versions. Create a compatibility matrix tracking: UI libraries, data fetching libraries, authentication packages, testing tools. Use a feature branch and test thoroughly in staging environments with production-like data volumes.

Key tasks: Run next codemod to automatically fix many breaking changes, audit all fetch() calls and add explicit cache directives, test with React 19 RC to identify incompatibilities, verify that your hosting provider supports Next.js 15 features (especially PPR).

Deploy to production with feature flags controlling new behaviors. Start with 5% traffic, monitoring error rates and performance metrics closely. For the e-commerce migration, we discovered that disabling aggressive caching improved data freshness but increased database load 3x. We added targeted caching for expensive queries, achieving a better balance than Next.js 14's blanket caching.

Monitor these metrics: database query rate, cache hit ratio, Time to First Byte (should improve with PPR), error rates (especially hydration mismatches), bundle sizes (should decrease with React 19 improvements).

Once stable, adopt Next.js 15-specific features: enable PPR for product/content pages, migrate background tasks to after() API, optimize caching strategies based on production telemetry, implement server actions for form handling (replacing API routes).

For a B2B SaaS application, this phase delivered measurable wins: 35% reduction in API response times, 20% decrease in infrastructure costs (fewer origin requests with strategic caching), improved SEO scores from faster Time to First Byte.