Migrating to Next.js for better performance and SEO

AutoZone completed a multi-year rebuild of its web app, migrating from a legacy stack to React with server-side rendering. However, after launch, the new platform experienced lower SEO performance compared to the previous version, hurting revenue and visibility during a time of critical algorithm changes at Google.

Key technical bottlenecks included:

• A custom SSR solution that limited flexibility and optimization
• A big single JavaScript bundle (~820kB), delaying interactivity
• Low PageSpeed Insights Performance scores, especially on mobile
• A slow feedback loop for developers, further delaying improvements

Callstack deployed a small, agile team onsite for a two-week deep dive to assess bottlenecks and prototype solutions. By measuring performance metrics and evaluating the development workflow, we identified key friction points and proposed a path forward.

Key actions included:

• Prototyping a Next.js migration to validate performance gains via code splitting, lazy loading and parallelization of network requests
• Estimating a realistic migration timeline with minimal disruption
• Spinning up a dedicated team focused solely on PageSpeed and SEO
• Using genetic algorithms to plan the migration in a conflict-free, parallelized way
• Improving dev feedback loops with Next.js Hot Module Replacement (and later, Fast Refresh)

Our close collaboration with the client team and fast iteration cycle helped align product and engineering around measurable goals.

The migration to Next.js and focus on improving performance quickly delivered tangible results, both for users and developers.

• Lighthouse Performance Score increase from 19 to 48 (mobile & desktop)
• Blocking JavaScript bundle size: from 820kB to 560kB
• 2–5x faster feedback loop for devs thanks to HMR and modern tooling
• Improved LCP, CLS, and TBT, directly impacting perceived speed

Search rankings rebounded as performance improved. AutoZone reclaimed its No. 1 position in key Google results for crucial keywords, restoring organic traffic and customer engagement without increasing marketing spend.

• Google Rank improvement form #3–9 to #1
• Faster UX, leading to better retention potential
• Developer velocity unlocked, enabling future optimization at scale