Core Web Vitals and Page Speed

Core Web Vitals are metrics defined by Google that reflect the user-friendliness of a website. These metrics can be used to measure and evaluate the user experience of a website by analyzing the loading process of a page, its interactivity and its visual stability.

LCP (Largest Contentful Paint):

Largest Contentful Paint refers to the point in time during the page load process when the main content is loaded and becomes visible to users. The largest element is usually an image, a video or a large block of text. LCP measures the render time of the largest visible image or text block in the viewport relative to when the page started loading. This is an important user-centric metric for measuring perceived load speed, as it marks the point in the page load timeline when the main content of the page is likely to have loaded. A fast LCP gives users the impression that the page is useful.

INP (Interaction to Next Paint):

Interaction to Next Paint measures the time it takes for a web page to visually respond to a user interaction. The measurement is made for each interactive element that the user clicks on. INP has replaced FID (First Input Delay) since March 2024. INP is an experimental metric for evaluating responsiveness. It observes the latency of all interactions of a user with the page and specifies a single value below which all (or almost all) interactions occurred. Observed interactions include mouse clicks, taps on a touchscreen and keystrokes on a physical or virtual keyboard.

CLS (Cumulative Layout Shift):

Cumulative Layout Shift quantifies how much the content of a page shifts visually during the loading phase.

The rating ranges from 0 to 1: zero means no shift and one means the greatest shift. This is important because the user experience suffers if page elements are shifted while users are trying to interact with them. CLS measures the largest instance of layout displacement that occurs over the lifetime of a page. Layout shifts can occur during the loading process or during page usage.

FID (First Input Delay):

First Input Delay measures the time it takes for a page to respond to the first user interaction. This was an important indicator until it was replaced by INP.

TBT (Total Blocking Time):

Total Blocking Time measures the total time during which a page is blocked and unable to respond to user interactions. It helps to identify problems with the interactivity of a page.

TTFB (Time to First Byte):

Time to First Byte is one of the “Other Web Vitals” as it is an important aspect of the loading process and can help diagnose LCP problems (slow server response times). There is now also a rating scale for the TTFB, although this specifies very “unambitious” target values.

Field data: This data comes from actual user visits to a website and includes different devices, network conditions and geographical locations. It is generated by real users and reflects the actual user experience. This data is also referred to as Real User Monitoring (RUM). Google uses the so-called CrUX data (Chrome User Experience Report), which is collected by the Chrome browser. Field data provides historical data from the last 28 days and is aggregated for the entire domain or per URL.

Lab Data: This data is collected in a controlled environment, also known as a “lab” or “synthetic” environment. It is collected under predefined network and device conditions and is used to verify issues and validate fixes during development. Lab data is useful for reproducing and fixing potential performance issues. This data is simulated and artificial and includes only one selected device, one network connection and one predefined geographic location per test run.

Lab Data is collected in a controlled environment using predefined device and network specifications. When measuring page performance, Lab Data is useful for reproducing and fixing potential performance issues.

Field Data reflects the browsing experience of real users using the website. It is dependent on the connection and the device they are using to browse. Field data is also referred to as Real User Monitoring (RUM). Google uses the so-called CrUX data (Chrome User Experience Report), which is collected by the Chrome browser.

1. User experience (UX): 88% of users do not return to a website after a bad experience.

2. Search engine optimization (SEO): Page speed and core web vitals are official ranking factors.

3. Conversion rates: Every second of delay in page load can reduce conversion rates by up to 20 percent.

4. Mobile user experience: Mobile users expect a website to load in less than 3 seconds.

5. Accessibility & inclusivity: Accessibility is already required by law (EAA). You can find specific recommendations for action and everything you need to know about the new regulations in our blog post on the topic of digital accessibility.

Core Web Vitals complement page speed - and vice versa. It is not technically possible to improve page speed without significantly optimizing the user experience. LCP, FID and CLS can hardly be manipulated. Nevertheless, you should not rely solely on the Core Web Vitals. In particular, Time To First Byte (TTFB) or Start Render are important metrics that the Core Web Vitals rely on. The Web Vitals are explicitly concerned with the user experience, while an improvement in page speed does not necessarily lead to a better UX.

Would you like to improve your customers' user experience and find out more about Core Web Vitals?