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Conversion rate optimization

Page Speed Optimization and Core Web Vitals (LCP, INP, CLS)

A slow page loses the visitor your ads paid for within the first few seconds. Google measures that experience with three Core Web Vitals: LCP for loading, INP for responsiveness and CLS for visual stability. Our page speed optimization doesn't chase a Lighthouse score of 100. The goal is to get real users into the 'good' range on all three metrics and to see that reflected in conversions.

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LCP2.1 s≤ 2.5 s goodINP180 ms≤ 200 ms goodCLS0,08≤ 0.1 goodCore Web Vitals
Illustrative view: illustrative view of Core Web Vitals metrics.
Purpose and appearance

What do we use this service for?

We work on page speed so that visitors brought in by ads and organic search aren't lost while the page loads. Improving LCP, INP and CLS supports mobile conversion rates and also contributes to landing page experience in search ads and to organic visibility. The goal is for more visitors to actually see the product page on the same ad budget.

Funnel stages it serves

  1. Awareness
  2. Consideration
  3. Conversion
  4. Loyalty

Core Web Vitals: field data vs. lab data (PageSpeed Insights and Lighthouse)

The Core Web Vitals report in Search Console and the top section of PageSpeed Insights are based on real data collected from Chrome users (CrUX) and reflect the last 28 days. Lighthouse, on the other hand, is a lab test run under a single device and connection assumption. Assessment is based on field data; the lab test is used to find the cause.

INP replaced FID in Core Web Vitals in March 2024 and can only be measured through user interaction. Since nobody clicks during a Lighthouse run, it doesn't produce INP; it shows Total Blocking Time (TBT) instead. That's why we investigate INP issues with field data and browser performance traces. Validating a fix in Search Console also requires a 28-day monitoring period, so we don't expect results the next day.

What slows ecommerce sites down: common LCP, INP and CLS issues

We rarely find the source of slowness in the server. It is usually in elements added to the page over the years: campaign widgets, pixels, live chat tools. On ecommerce sites in Türkiye, where mobile traffic dominates, this load is far more visible on mid-range phones. Each element looks small on its own; together they add up. The causes we see most often, by metric:

  • LCP: An auto-rotating campaign slider on the homepage; the first image loads late, and lazy loading has mistakenly been applied to it too
  • LCP: Product images served as large JPEGs instead of WebP or AVIF, at desktop size on mobile
  • INP: Live support, chat, multiple ad pixels and behavior analytics scripts keeping the main thread busy
  • INP: Heavy JavaScript in interactions such as filtering and variant selection
  • CLS: A late-loading promo bar, cookie banner or images without set dimensions pushing content down
  • CLS: Text reflowing while the web font loads

How we prioritize Core Web Vitals fixes

We start with the templates where traffic and revenue concentrate: homepage, category, product and checkout. A fix to the product template improves thousands of pages at once. We give your developers the list with technical notes showing what changes in which file, such as using fetchpriority on the LCP image or setting width and height on images.

With third-party code, the biggest gain usually comes from moving to server-side tagging and removing unused tags. Speed is also an important signal for technical SEO, and in Search campaigns landing page experience affects Quality Score. We decide together which tags to remove by checking them against the list of third-party tools on the site and their contract owners.

Page Speed and Core Web Vitals: when do we recommend it?

We recommend it

  • Sites whose mobile URLs show as 'poor' or 'needs improvement' in Search Console
  • Ecommerce sites that have built up rotating banners, chat tools and many pixels on their homepage and product templates
  • Sites that slowed down after adding a new theme, plugin or third-party tool

We don't recommend it

  • Spending development time just to raise the Lighthouse score when field data is already 'good'
  • Prioritizing speed when the real cause of lost conversions is price or the checkout flow

How we manage Page Speed and Core Web Vitals

  1. Field dataWe pull problematic URL groups for mobile and desktop from Search Console and CrUX data.
  2. Template selectionWe prioritize the page templates with the highest traffic and revenue.
  3. Root cause analysisUsing Lighthouse and browser performance traces, we find the LCP element, long tasks and sources of layout shift.
  4. Technical notesWe give developers a list of fixes at file and element level.
  5. 28-day monitoringAfter release, we track field data and conversion rate together.

Page Speed and Core Web Vitals: metrics and requirements

Metrics we track

  • LCP (75th percentile)
  • INP (75th percentile)
  • CLS (75th percentile)
  • Share of good URLs
  • Mobile conversion rate

What you need to get started

  • Access to the Search Console property
  • A developer, or support from your platform provider, to implement fixes
  • A list of third-party tools running on the site and their contract owners
  • A staging environment or pre-release testing option

Official sources

The Brand Move CRO Team

To learn more about Page Speed and Core Web Vitals or other conversion optimization services, contact The Brand Move Conversion Optimization team today.

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Frequently asked questions

Page Speed and Core Web Vitals: frequently asked questions

What are Core Web Vitals?

Core Web Vitals are three metrics Google uses to measure page experience: LCP (Largest Contentful Paint) for loading, INP (Interaction to Next Paint) for responsiveness and CLS (Cumulative Layout Shift) for visual stability. INP replaced FID in March 2024. Assessment is based on real Chrome user data from the last 28 days.

PageSpeed Insights gives us a mobile score of 40. Is that bad?

On its own, it doesn't tell you much. The score comes from a lab test that assumes a slow mobile device. What really matters is the real-user data at the top of the same page; if LCP, INP and CLS are 'good' there, your users are getting a fast experience even if the score is low.

How do you fix a poor INP score?

Lighthouse can't measure INP because nobody clicks during the test, so we work from field data and browser performance traces. The usual causes are third-party scripts keeping the main thread busy and heavy JavaScript in filters or variant selection. Moving tags to server-side tagging and removing unused ones often brings the biggest gain.

Does page speed affect SEO and Google Ads?

Yes. Speed is a signal in technical SEO, and in Search campaigns landing page experience feeds into Quality Score. Validating a fix in Search Console takes a 28-day monitoring period, so results don't show the next day.

We're on a hosted ecommerce platform. What can be done about speed?

The server and core code belong to the platform provider, but images, theme files, plugins and Tag Manager content are mostly under your control. Most of the gain comes from these areas; platform-level issues we raise with the provider, backed by technical evidence.

How do you measure the impact of speed fixes on sales?

Because we release changes template by template and in sequence, we compare before and after using the same template's conversion rate. To separate out seasonality and campaign effects, we monitor templates that weren't fixed in the same period as a control group.

Let's run a first Page Speed and Core Web Vitals review together.

In a free call we review the drop-off points in your funnel and your landing pages, then send you the first three ideas to test in writing.

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