Web core vitals

Web Core Vitals are measured in the following important parameters.

  • Largest Contentful Paint (LCP)

  • First Input Delay (FID)

  • Total Blocking Time (TBT)

  • Cumulative Layout Shift (CLS)

  • First Contentful Paint (FCP)

  • Time to Interactive (TTI)

  • Speed Index

Largest Contentful Paint (LCP)

Largest Contentful Paint marks the time at which the largest text or image is painted.

How lighthouse determines LCP?

LCP time (in seconds) Color Coding
0 - 2.5 Green (fast)
2.5 - 4 Orange (moderate)
Over 4 Red (slow)

How to improve LCP?

  • Reduce CSS Blocking Time

  • Reduce JavaScript Blocking Time

Reduce CSS blocking time

1. Minify CSS: For better understanding, CSS files can contain characters such as spacing, indentation, or comments. These characters are all unnecessary for the browser, and minifying these files will ensure that they will get removed. Remove any unused CSS entirely or move it to another stylesheet if used on a separate page of your site.

2. Inline Critical CSS: Inline any CSS used for above-the-fold content by including it directly in "head" tag.

Reduce javaScript blocking time

Download the javascripts like bootstrap, jquery etc rather than directly fetching from the CDN links. Now combine all the javascripts and minify the file.

Improve web core vitals score by Lazy loading javaScript ➞

First input delay (FID)

FID measures interactivity. To provide a good user experience, pages should have a FID of 100 milliseconds or less. Some of the google tools use TBT instead of FID. To predict FID, Total Blocking Time (TBT) is recommended. They measure different things, but improvements in TBT usually correspond to improvements in FID.

Total Blocking Time (TBT)

TBT measures the total amount of time that a page is blocked from responding to user input, such as mouse clicks, screen taps, or keyboard clicks. Any task that executes for more than 50 ms is a long task. The amount of time after 50 ms is the blocking portion.

How lighthouse determines TBT?

TBT time (in milliseconds) Color Coding
0 - 300 Green (fast)
300 - 600 Orange (moderate)
Over 600 Red (slow)

How to improve TBT?

Unnecessary JavaScript loading. Remove all the unnecessary javascript to improve TBT.

Cumulative Layout Shift (CLS)

CLS measures visual stability. Unexpected movement of page content usually happens because resources are loaded asynchronously or DOM elements get dynamically added to the page above existing content.

How lighthouse determines CLS?

CLS Status
≤0.1 Good
>0.25 Poor

How to improve CLS?

1. Use size attribute dimensions for any media: Always include size attributes on your images and video elements. This will assign a certain amount of space until the image loads.

2. Reserve space for ad elements: Make sure ads elements have a reserved space. Otherwise they can suddenly appear on the page, pushing content down, up or to the side.

Improve web core vitals score by lazy loading images ➞

First Contentful Paint (FCP)

FCP measures how long it takes the browser to render the first piece of DOM content after a user navigates to your page. Images, "canvas" elements, and SVGs on the page are considered DOM content. Anything inside an iframe is not included.

How lighthouse determines FCP?

FCP time (in seconds) Color Coding
0 - 2 Green (fast)
2- 4 Orange (moderate)
Over 4 Red (slow)

How to improve FCP?

One issue that is important for FCP is font load time. Load the fonts from the local storage i.e., Download the fonts and use them using @font-face instead of giving the CDN links.

Time to Interactive (TTI)

TTI measures how long it takes a page to become fully interactive. A page is considered fully interactive when:

  • The page displays useful content, which is measured by the First Contentful Paint

  • The page responds to user interactions within 50 milliseconds

How lighthouse determines TTI?

TTI time (in seconds) Color Coding
0 - 3.8 Green (fast)
3.9 - 7.3 Orange (moderate)
Over 7.3 Red (slow)

How to improve TTI?

1. Minimize main thread work: Optimize third-party javascripts.

1. Reduce JavaScript execution time: Remove unused scripts which are not required for the website.

Speed index

Speed Index measures how quickly content is visually displayed during page load. Lighthouse captures a video of the page loading in the browser and computes the visual progression between frames.

How lighthouse determines speed index?

Speed Index (in seconds) Color Coding Score
0 - 4.3 Green (fast) 75 - 100
4.4 - 5.8 Orange (moderate) 50 - 74
Over 5.8 Red (slow) 0 - 49

How to improve speed index?

  • Minimize main thread work

  • Reduce JavaScript execution time

  • Ensure text remains visible during webfont load

Read Next: Improve web core vitals score by reducing server response time ➞

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