Solving Core Web Vitals Challenges in Single Page Apps

Ritisha

November 11, 2025

Modern web experiences are built on speed, interactivity, and seamless design. Today, Single Page Applications (SPAs) have become the go-to architecture for delivering such experiences. From SaaS dashboards to e-commerce platforms, SPAs offer fluid navigation and app-like responsiveness.

But with great flexibility comes great responsibility. Search engines and users alike demand fast, stable, and responsive websites — and this is where Core Web Vitals (CWV) come into play. Introduced by Google as part of its Page Experience update, Core Web Vitals are measurable signals that define how users perceive the speed and usability of a website. They directly influence both SEO rankings and user satisfaction.

Here’s the challenge: while traditional websites often struggle with optimization, SPAs face unique hurdles. Because of their reliance on JavaScript-heavy rendering and client-side routing, SPAs frequently lag behind in metrics like Largest Contentful Paint (LCP), Interaction to Next Paint (INP, formerly FID), and Cumulative Layout Shift (CLS).

In this blog, we’ll explore why SPAs find Core Web Vitals difficult to pass, uncover strategies for overcoming these obstacles, and provide a roadmap for developers to optimize their single page web apps without sacrificing interactivity or SEO.

Understanding Core Web Vitals in the Context of SPAs

Before diving into solutions, let’s establish the basics.

What are Core Web Vitals?

Core Web Vitals are three primary performance metrics defined by Google:

• Largest Contentful Paint (LCP): Measures loading performance. The goal is to render the largest visible element (text, image, or video) within 2.5 seconds.
• Interaction to Next Paint (INP): Replaces First Input Delay (FID) as a measure of responsiveness. It records how quickly the page responds to user interactions.
• Cumulative Layout Shift (CLS): Measures visual stability by tracking unexpected layout movements during page load.

Why SPAs Are Different

Unlike traditional multi-page applications (MPAs), which reload HTML on each navigation, SPAs load a single HTML shell and dynamically update content using JavaScript. While this creates smoother transitions and fewer reloads, it also shifts much of the workload to the browser.

As a result:

JavaScript execution delays rendering of critical content, harming LCP

When SPAs rely heavily on JavaScript, the browser must process large files before showing important content. This delays Largest Contentful Paint, making pages feel slower and less responsive for users.

Heavy scripts block the main thread, slowing interaction responsiveness

Large or unoptimized JavaScript tasks keep the browser’s main thread busy. This means user interactions, like clicks or taps, feel delayed, leading to poor Interaction to Next Paint scores and weaker user experiences.

Client-side rendering causes unstable layouts, increasing CLS

In SPAs, layouts often shift as content loads dynamically. Missing image dimensions, late font loading, or injected elements push content around, raising Cumulative Layout Shift scores and frustrating visitors with unstable visuals.

Search engines may struggle with crawling and indexing SPA content, impacting single page application for SEO

Since SPAs often load content with JavaScript, crawlers may not see important text or links. Without prerendering or SSR, search engines index poorly, weakening SEO performance and discoverability.

SPAs deliver powerful UX, but without careful optimization, they often underperform in the very metrics Google prioritizes for rankings.

Common Core Web Vitals Challenges in SPAs

SPAs tend to hit the same roadblocks repeatedly when it comes to CWV. Let’s break them down:

1. Delayed Largest Contentful Paint (LCP)

SPAs often rely on large JavaScript bundles to render core components. This means users see a blank screen or a loading spinner until hydration is complete. The result: LCP happens much later than on a server-rendered page.

2. Poor Interaction to Next Paint (INP)

INP measures how quickly a page reacts to a tap, click, or key press. In SPAs, scripts run on the main thread, often creating long tasks that block responsiveness. For users, this translates into sluggish interactions.

3. High Cumulative Layout Shift (CLS)

Since SPAs load content dynamically, layouts can shift unexpectedly. Common culprits include:

• Images loading without defined dimensions.
• Fonts swapping after render.
• Ads or widgets injecting into the DOM after initial load.

4. SEO Discoverability Issues

Search engines rely on crawling and indexing to rank pages. But in SPAs, content often requires JavaScript execution to appear. Without server-side rendering or prerendering, crawlers may see an empty page. This weakens the single page application for SEO strategy.

5. Client-Side Routing and Metrics Tracking

In SPAs, page transitions don’t reload the entire document. This makes it harder for monitoring tools (and even Google) to track metrics like LCP on navigations beyond the initial page load.

Why SPAs Struggle More Than Traditional Websites

SPAs bring architectural strengths but performance weaknesses. Here’s why:

• Render-blocking JavaScript: SPAs heavily depend on JS for everything from rendering to routing. Without optimization, scripts delay the time-to-content.
• Hydration bottlenecks: Frameworks like React or Angular must hydrate components client-side before becoming interactive, adding delays.
• Client-side routing: Navigating inside an SPA does not trigger a full reload. While fast for users, it complicates CWV measurements.
• Caching complexity: Browser caches work best with static assets. SPAs often generate dynamic bundles, reducing caching efficiency.
• Complex SEO signals: Since content is dynamically loaded, metadata and canonical tags may not appear in the HTML by default, hurting discoverability.

In essence, SPAs trade off speed of interaction for flexibility — but with the right approach, both are possible.

Proven Strategies to Improve Core Web Vitals in SPAs

Optimization is not about abandoning SPAs but making them smarter. Let’s explore strategies aligned with each Core Web Vital.

1. Optimizing Largest Contentful Paint (LCP)

• Server-Side Rendering (SSR): Rendering content on the server ensures users see meaningful content faster. Frameworks like Next.js or Nuxt.js excel here.
• Static Site Generation (SSG): For content that doesn’t change often, prerendering pages at build time accelerates delivery.
• Code Splitting & Lazy Loading: Split large JS bundles into smaller chunks. Load only what’s needed for the initial view.
• Preloading Key Assets: Fonts, hero images, and CSS files should be preloaded to minimize render delays.

2. Improving Interaction to Next Paint (INP)

• Break Down Long Tasks: Use requestIdleCallback or scheduling to split intensive scripts into smaller tasks.
• Web Workers: Offload heavy computations from the main thread.
• Optimize Event Handlers: Avoid attaching complex logic to user interactions. Keep handlers lightweight.
• Prioritize Critical Interactivity: Ensure that UI controls respond immediately, even if secondary scripts load later.

3. Fixing Cumulative Layout Shift (CLS)

• Reserve Space for Media Elements: Always define width and height for images and video placeholders.
• Preload Fonts: Avoid “flash of invisible text” or font swaps.
• Stabilize Dynamic Content: Ensure ads or third-party widgets have fixed containers.
• Optimize Rendering Logic: Avoid adding content above already loaded elements.

4. Enhancing Crawlability & SEO for SPAs

• Hybrid Rendering Approaches: Combine SSR with client-side hydration for the best of both worlds.
• Prerendering for Bots: Tools like Prerender.io or Rendertron serve static HTML snapshots for crawlers.
• Structured Data & Canonical Tags: Ensure Googlebot gets the correct metadata, even for SPAs.
• SEO-Friendly Routing: Use descriptive, crawlable URLs instead of hash-based routing.

Advanced Techniques for SPA Performance Optimization

For developers ready to push further:

Streaming Rendering (React 18, Next.js)

Streaming rendering sends HTML to the browser in small parts instead of waiting for the entire page to finish. This allows users to see important content faster, improving Core Web Vitals by reducing Largest Contentful Paint delays significantly.

Suspense for Data Fetching

With React Suspense, critical above-the-fold content loads first while less important data is fetched in the background. This makes the page feel instantly usable, improving responsiveness and keeping users engaged while other information loads without slowing interaction.

API Response Caching

API response caching stores frequently used data on CDNs or edge servers closer to users. This reduces latency, speeds up content delivery, and avoids repeated server calls, helping Single Page Applications improve loading performance and overall Core Web Vitals scores.

Core Web Vitals Monitoring

Implementing the Web Vitals JavaScript library allows developers to capture real-world performance metrics like LCP, INP, and CLS. By monitoring these numbers continuously, developers can detect performance issues early and optimize SPAs for better SEO, usability, and rankings.

Best Practices Checklist for Developers

Use SSR or SSG where possible

Render content on the server or during build time so users instantly see main content without waiting for scripts.

Minimize JavaScript bundle sizes

Reduce unnecessary JavaScript by removing unused code and libraries so pages load faster and users get content quickly.

Split code and lazy load non-critical components

Break large files into smaller chunks and load only what’s needed first, keeping the rest for when users need it.

Preload fonts and hero images

Tell the browser to download important fonts and top images early, so main page elements appear quickly and clearly.

Reserve dimensions for all media

Always set width and height for images, videos, and ads so layouts don’t jump or shift while loading.

Optimize routing for SEO and CWV tracking

Use clean, crawlable URLs and ensure Core Web Vitals are tracked correctly across different routes in your SPA.

Continuously monitor metrics with Lighthouse, PageSpeed Insights, or Chrome UX Report

Regularly test performance using these tools to spot problems early and keep improving your site’s speed and stability.

Conclusion

Single Page Web Applications are here to stay, offering users fast, dynamic, and app-like experiences. However, when it comes to Core Web Vitals, SPAs face inherent hurdles that can impact both user satisfaction and SEO rankings.

By adopting server-side rendering, optimizing JavaScript execution, stabilizing layouts, and enhancing crawlability, developers can strike a balance between interactivity and performance. The goal isn’t to abandon SPAs but to build them with a user-first approach where speed and stability are just as important as design and features.

If your SPA struggles with Core Web Vitals and SEO, expert guidance can make all the difference. Hire Core Web Vitals Consultants to ensure your website not only delights users but also performs strongly in search rankings.