If there were a single technical change that could simultaneously improve your Google rankings, reduce your bounce rate, and increase the percentage of visitors who convert — most businesses would make that change immediately. Core Web Vitals is that change. And yet the majority of websites, including many that actively invest in SEO, still fail one or more of the three metrics Google uses to measure page experience.
This guide explains what each metric actually measures, what changed in 2024 that many websites have still not addressed, and the priority order for fixes that produce the biggest improvement with the least effort.
What Core Web Vitals Are and Why Google Uses Them
Core Web Vitals are real-world performance metrics collected from Chrome browser data across millions of actual user page visits — not laboratory simulations run on ideal connections. Google introduced them as a ranking signal through the Page Experience update in 2021, and they have grown in ranking importance since then.
The logic is straightforward: Google’s goal is to direct users toward pages that give them a genuinely good experience. A page that takes five seconds to show its main content, shifts layout unexpectedly while loading, or responds sluggishly to clicks is a poor experience. Google prefers to send users somewhere else. The relationship between speed and conversion rates is well documented — poor Core Web Vitals hurt both your rankings and your revenue simultaneously.
There are three metrics. Your site needs to pass all three to achieve a “Good” page experience rating.
The Three Metrics Explained
Largest Contentful Paint (LCP)
LCP measures how long it takes for the largest visible content element on a page to fully render. This is usually a hero image, a large text heading, or a main content block — whatever is the biggest element in the initial viewport when the page loads.
Under 2.5 seconds is “Good.” Between 2.5 and 4 seconds is “Needs Improvement.” Over 4 seconds is “Poor.” LCP has the strongest correlation with perceived page speed and the most direct connection to ranking impact of the three Core Web Vitals. Most LCP problems are caused by large unoptimised images or render-blocking scripts that delay how quickly main content can appear.
Interaction to Next Paint (INP)
INP replaced First Input Delay in March 2024 — and this is the change most websites have not fully addressed. Where FID only measured the delay on a user’s very first interaction with a page, INP measures the responsiveness of every interaction throughout the entire page session — every click, tap, and keyboard input — and captures the worst-case delay experienced during a visit.
Under 200 milliseconds is “Good.” Between 200 and 500 milliseconds is “Needs Improvement.” Over 500 milliseconds is “Poor.” Many sites that were passing FID are now failing INP. INP problems are almost always caused by heavy JavaScript execution — third-party scripts, complex interactive components, and poorly optimised event handlers all block the browser’s main thread when a user tries to interact.
Cumulative Layout Shift (CLS)
CLS measures visual stability — how much the visible page layout moves unexpectedly while loading. You have experienced this when you go to click a button and an image loads above it, pushing the button down just as you tap. Or when text reflows as a custom font finishes loading. These unexpected visual movements are frustrating for users and are measured and penalised by Google.
Under 0.1 is “Good.” Between 0.1 and 0.25 is “Needs Improvement.” Over 0.25 is “Poor.” CLS problems are typically caused by images without defined width and height attributes, ads or content blocks injected into the page after initial load, or web fonts causing text reflow when they arrive.
How Core Web Vitals Function as a Ranking Signal
Core Web Vitals function more as a floor threshold and a tiebreaker than as a dominant factor that overrides content relevance or backlink authority. When two pages are equally relevant and have similar authority, the one with better Core Web Vitals scores will tend to rank higher. Pages failing significantly — especially LCP over 4 seconds — can experience meaningful ranking suppression relative to competitors with better scores in the same category.
The signal uses real-world field data from the Chrome User Experience Report — not lab test results. A page that passes PageSpeed Insights in a simulated test can still fail in field data if real users are on slower mobile connections or older devices. It is the field data that matters for rankings.
What Is Most Likely Causing Your Site to Fail
LCP Problems
Unoptimised hero images are by far the most common cause of poor LCP scores. A homepage banner uploaded at 3,000 pixels wide and 2MB in file size, displayed at 1,200 pixels in the browser, is delivering over twice the data necessary. Every above-the-fold image — particularly hero banners and featured images — should be compressed, resized to display dimensions, and served in WebP format. WebP files are typically 25–35% smaller than equivalent JPEGs at the same visual quality.
Render-blocking resources are the second major cause. CSS and JavaScript that must be processed before the browser can render anything visible delay the moment at which LCP content appears. Critical CSS should be inlined in the page head. Non-critical scripts should be deferred or loaded asynchronously. Server response time is also a factor — if your Time to First Byte is above 600 milliseconds, hosting performance may be the bottleneck, and no amount of front-end optimisation will fully compensate.
INP Problems
Third-party scripts are the primary cause of poor INP scores. Every tracking pixel, live chat widget, cookie consent tool, A/B testing script, and advertising tag runs on the browser’s main thread. When a user tries to click or interact with your page, all of these scripts compete for the main thread’s attention, creating the delays that INP measures.
Audit your third-party scripts regularly. Remove anything you are not actively using. Load what you do need asynchronously or after the page has become interactive. The INP improvement from removing just two or three unused third-party scripts is often dramatic. Long JavaScript tasks — individual scripts that execute for more than 50 milliseconds — should be broken into smaller chunks that yield to the browser between executions.
CLS Problems
Images without defined dimensions are the most common cause of layout shift. When a browser loads an image but does not know its height and width in advance, it cannot reserve space for it — so when the image loads, it pushes content below it downward. Adding explicit width and height attributes to every image tells the browser how much space to reserve before the image arrives.
Ads and content embeds that inject blocks above existing content after the page has loaded are the other major CLS cause. Where possible, reserve explicit space for ad slots even when no ad fills them, and use skeleton placeholders for content loaded asynchronously.
How to Check Your Scores
Google PageSpeed Insights at pagespeed.web.dev gives you both lab simulation results and real-world field data for any URL you enter, along with specific diagnostic recommendations for each issue found. Google Search Console’s Core Web Vitals report shows aggregated field data across your entire site broken down by URL groups — this is the most important view because it shows how your pages perform for real users, which is what Google uses for rankings.
Check both tools. A URL that passes in PageSpeed Insights can still fail in Search Console field data if real user traffic shows poor performance at scale. Address any URLs flagged as “Poor” or “Needs Improvement” in Search Console as a priority over optimising pages that are already passing.
Priority Order for Fixes
Fix in this sequence for maximum ranking and user experience improvement per unit of effort. First: compress and resize all above-the-fold images and convert to WebP — this typically produces the largest single LCP improvement and can be done quickly. Second: add explicit width and height attributes to all images — fixes the majority of CLS issues site-wide. Third: audit and remove all unused third-party scripts — the fastest path to INP improvement and often the most impactful change. Fourth: defer non-critical JavaScript. Fifth: enable server-side caching. Sixth: if TTFB is above 600ms, evaluate your hosting or implement a CDN.
Most sites see measurable LCP and CLS improvements within days of implementing the first two steps. INP improvements from script removal are immediate upon deployment. If your site has been producing good content and building links but rankings have plateaued, failing Core Web Vitals is one of the most common suppressing technical factors — and one of the most directly fixable. Our technical SEO audit includes a full Core Web Vitals assessment with specific URL-level recommendations as part of every engagement.