Why Is My Website Still Failing Core Web Vitals After a Full Website Redesign?

I recently completed a full website redesign, expecting better speed and SEO performance, but my site is still failing Core Web Vitals. What could be causing this, and what should I check to improve my scores?

One of the biggest misconceptions I encounter as an SEO specialist is the belief that a website redesign automatically improves performance. During my three years working at a digital marketing agency in Birmingham, I've seen several UK businesses invest heavily in redesigns only to discover that their Core Web Vitals scores remained poor or even declined after launch.

Core Web Vitals are not based on appearance. Google measures how real users experience your website through metrics such as:

• Largest Contentful Paint (LCP): Loading performance
• Interaction to Next Paint (INP): Responsiveness to user interactions
• Cumulative Layout Shift (CLS): Visual stability

A redesigned website may look modern, but if the underlying technical issues remain unresolved, performance scores will continue to suffer.

Real User Data Often Delays Improvements

One of the first things I check when clients report poor Core Web Vitals after a redesign is the source of the data.

Google Search Console relies on real-world Chrome user data collected over approximately 28 days. This means that even after significant improvements, historical poor performance may continue appearing in reports for several weeks.

I have worked with multiple UK businesses that assumed their redesign had failed when, in reality, Google was still evaluating older user experience data.

Heavy Visual Design Can Hurt Largest Contentful Paint (LCP)

A common issue I see is oversized visual assets introduced during redesign projects.

One Birmingham-based professional services client replaced a simple homepage banner with:

• Full-screen hero images
• Background videos
• Animated sliders
• Multiple custom fonts

The result was an LCP exceeding four seconds.

The most common causes of poor LCP include:

• Uncompressed images
• Large hero banners
• Background videos
• Excessive web fonts
• Render-blocking CSS
• Slow server response times

In most cases, image optimisation, font loading improvements, and asset prioritisation significantly reduce loading times.

Excessive JavaScript Is Often the Hidden Problem

Modern websites frequently rely on JavaScript-heavy frameworks. While these frameworks provide rich user experiences, they can negatively affect performance if not properly managed.

I have seen redesigns where the website visually appeared fast, but user interactions felt sluggish because browsers were processing large JavaScript files behind the scenes.

Common JavaScript-related issues include:

• Large JavaScript bundles
• Unused JavaScript code
• Client-side rendering
• Heavy page builders
• Excessive animations

When JavaScript execution becomes excessive, Interaction to Next Paint (INP) often suffers.

Third-Party Scripts Can Destroy Performance

One of the most overlooked causes of poor Core Web Vitals is third-party software.

Several UK clients I have worked with unknowingly introduced performance issues through marketing tools rather than website design itself.

Typical offenders include:

• Live chat software
• Cookie consent platforms
• Analytics tools
• A/B testing software
• Social media widgets
• Heatmap tracking solutions

These external scripts often load from third-party servers, making performance unpredictable and difficult to control.

Poor Hosting Can Limit Every Optimisation

Many businesses focus exclusively on front-end improvements while ignoring server performance.

I have worked on websites where:

• Images were optimised
• Code was minimised
• Caching was configured

Yet the site still failed Core Web Vitals because the hosting environment was slow.

Common server-side problems include:

• Shared hosting limitations
• Slow database queries
• High Time to First Byte (TTFB)
• Poor CDN configuration
• Inefficient backend processing

In some projects, upgrading hosting infrastructure produced greater improvements than months of front-end optimisation.

Layout Shift Issues Often Appear After Launch

Cumulative Layout Shift (CLS) problems frequently emerge after a redesign because visual elements are loaded dynamically.

Some common causes include:

• Images without dimensions
• Late-loading advertisements
• Font swapping
• Cookie banners appearing above content
• Dynamically injected elements

I often see websites pass initial testing but develop CLS issues once marketing tools and advertising components are activated on the live environment.

Unused CSS Can Slow Rendering

Many redesigned websites use premium themes or page builders containing thousands of lines of CSS that are never actually used.

This creates render-blocking resources that delay visible content.

Typical CSS-related issues include:

• Large framework files
• Unused stylesheets
• Multiple CSS libraries
• Blocking critical rendering paths

Removing unused CSS and prioritising critical styles can have a noticeable impact on performance metrics.

Caching and CDN Configuration Are Frequently Overlooked

Another recurring issue I encounter involves websites launching without proper caching strategies.

After redesign projects, I always verify:

• Browser caching
• CDN implementation
• Gzip or Brotli compression
• Static asset caching
• Cache-control headers

Even well-designed websites can perform poorly if these technical fundamentals are missing.

What I Usually Check First

When auditing a redesigned website that still fails Core Web Vitals, I typically investigate:

• Google Search Console Core Web Vitals report
• PageSpeed Insights diagnostics
• Chrome DevTools performance analysis
• Server response times
• Third-party script impact
• Image optimisation status
• JavaScript execution time
• CSS efficiency
• CDN and caching configuration

Final Thoughts

Based on my experience working with businesses across Birmingham and the wider UK, failing Core Web Vitals after a redesign is rarely caused by a single issue. More often, it results from a combination of heavy media files, excessive JavaScript, third-party tools, hosting limitations, inefficient CSS, and poor caching practices.

A redesign changes how a website looks. Core Web Vitals measure how efficiently it performs in real-world conditions. The businesses that achieve the best results are those that treat performance optimisation as an ongoing technical process rather than a task that ends when the new website goes live.