Should you really split your JavaScript bundles by site section for better SEO?

Why does Google insist on splitting JavaScript bundles?

The problem of monolithic bundles has been known for years: a single 500 KB file that contains the logic for the blog, forum, shop, and back office. The result? Every page of the site loads code that is irrelevant to it. The Googlebot crawler downloads this heavy bundle on every visit, wasting time and crawl budget, and users suffer from latency.

Martin Splitt claims that splitting by logical sections solves several problems at once. A visitor navigating the blog only downloads the blog bundle. If they switch to the shop, the blog bundle remains cached, and only the shop bundle is loaded. Changes in one section only force the re-download of that specific bundle, not the entire site's JavaScript.

What's the direct connection to technical SEO?

For Googlebot, every millisecond counts. A massive single bundle slows down the rendering time, delays content extraction, and can even cause timeouts on complex pages. By segmenting bundles, we reduce the JavaScript weight per page, speed up Time to Interactive, and potentially improve the Core Web Vitals signals (LCP, FID, CLS).

The cache also plays a crucial role. Googlebot caches JavaScript resources between visits. With a bundle per section, a minor change in the forum doesn't invalidate the cache of the blog or shop. In practical terms: fewer re-downloads, less bandwidth consumed on Google's side, more pages crawled with the same crawl budget.

What does “logical sections” really mean?

This isn't about blindly splitting the code into 50 micro-bundles. A logical section corresponds to a part of the site with distinct functionalities: the blog with its comments and sharing system, the shop with the cart and checkout, the forum with its moderation and notification system.

The idea is to group functionalities that naturally coexist while avoiding unnecessary cross-dependencies. If the blog and shop share a UI component library, it can reside in a shared common bundle, loaded once and cached for the entire site. The rest of the code specific to each section remains isolated.

• Reduction of JavaScript weight per page: only the necessary bundles are loaded
• Improved browser and Googlebot caching: changes in one section don't invalidate other bundles
• Positive impact on Core Web Vitals: potentially improved Time to Interactive and First Input Delay
• Optimization of crawl budget: less wasted bandwidth, more pages crawled
• Logical separation of code: facilitates maintenance and deployment by the team

Is this recommendation really applicable to all sites?

On paper, Martin Splitt's advice is sound. In practice, the feasibility depends heavily on the front-end architecture in place. A site that uses a Single Page Application (SPA) with a framework like React or Vue will naturally load a heavy initial bundle, then lazily load pieces on the fly. Splitting by section in this context requires a redesign of routing and chunking, which is not trivial.

Sites with server-side rendering (SSR) or Static Site Generation (SSG) are better off: each section can have its own JavaScript entry point without touching the rest. But even there, it’s crucial to manage shared dependencies without duplicating code. If each bundle contains its own copy of Lodash or Axios, total weight hasn’t been reduced at all.

Does Google provide enough details to take action?

Let's be honest: [To be verified] Splitt's statement remains generic. What is the ideal size for a segmented bundle? At what number of kilobytes does one need to split? How many distinct bundles can one have before the number of HTTP requests becomes counterproductive, even with HTTP/2? No quantified metrics are provided.

We know that Googlebot supports HTTP/2 and that multiple requests weigh less than before. But real performance depends on network latency, SSL certificate, negotiation time. A site that goes from 1 bundle of 400 KB to 8 bundles of 50 KB may see its First Contentful Paint degrade if the bundles are loaded sequentially without intelligent preloading. Google does not specify anything about optimal loading order.

What are the risks of applying this advice without measurement?

The main pitfall: excessive fragmentation. One can end up with 30 micro-bundles that load in succession, each waiting on a dependency of the other. Request waterfalls lengthen, Time to Interactive skyrockets, and you end up with a site slower than it was before splitting.

Another risk: code duplication between bundles. If both the blog bundle and the shop bundle contain the same form validation library, the total weight downloaded by a user navigating between the two sections is greater than it would have been with a well-optimized single bundle. Therefore, it is imperative to extract shared dependencies into a shared chunk.

How do you concretely split your JavaScript bundles by section?

The first step is to map out the logical sections of your site. Identify the major functional areas: blog, shop, user area, forum, landing pages. Each of these sections has distinct JavaScript needs. The blog requires comments and social sharing, the shop needs a cart and checkout, the user area needs authentication and user profiles.

Next, analyze the shared dependencies. Which modules are shared between multiple sections? UI libraries (buttons, modals, tooltips), utilities (date, validation, formatting), and analytics trackers should live in a shared bundle. Webpack, Rollup, or Vite allow you to configure split points and shared chunks to automatically extract this common code.

What mistakes should be avoided when splitting bundles?

Do not split without measuring. Use Lighthouse and WebPageTest to capture metrics before splitting: Time to Interactive, Total Blocking Time, number of requests, total weight transferred. After splitting, compare these metrics on a page-by-page basis. If TTI increases or the number of requests spikes, then the splitting is too granular.

Avoid cross-dependencies between section bundles. If the blog bundle calls code from the shop bundle, you've created a hidden dependency that forces loading both bundles when just one would suffice. Keep each bundle self-contained, with only dependencies towards the shared common bundle.

How to verify that splitting actually improves SEO?

Monitor the crawl data in Search Console. Compare the number of pages crawled per day before and after splitting. If Googlebot crawls significantly more pages with the same frequency, it means the crawl budget is better utilized. Also check for JavaScript timeout errors: these should decrease.

Regarding Core Web Vitals, track the evolution of the Largest Contentful Paint and First Input Delay in the CWV report from Search Console. An improvement in these metrics often translates to better rankings, especially on mobile where bandwidth is limited and JavaScript is more costly in CPU time.

• Map out the logical sections of the site and their distinct JavaScript needs
• Identify the shared dependencies and extract them into a shared bundle
• Configure the bundler (Webpack, Rollup, Vite) with split points by section
• Measure Core Web Vitals before/after using Lighthouse and WebPageTest
• Monitor crawl budget and timeout errors in Search Console
• Verify that the total weight transferred actually decreases for each type of page