Each Performance audit that contributes to your score has its own scoring methodology. Lighthouse maps each raw score to a number between 0 and 100. The scoring distribution is a log normal distribution derived from the performance metrics of real website performance data on HTTPArchive.
Lighthouse returns a Progressive Web App (PWA) score between 0 and 100. 0 is the worst possible score, and 100 is the best.
The PWA audits are based on the Baseline PWA Checklist, which lists 14 requirements. Lighthouse has automated audits for 11 of the 14 requirements. The remaining 3 can only be tested manually. Each of the 11 automated PWA audits are weighted equally, so each one contributes approximately 9 points to your PWA score.
The Accessibility score is a weighted average of all the accessibility audits. See Scoring Details for a full list of how each audit is weighted. The heavier-weighted audits have a bigger impact on your score.
Lighthouse returns a Best Practices score between 0 and 100. 0 is the worst possible score, and 100 is the best.
The Best Practices audits are equally weighted. To calculate how much each audit contributes to your overall Best Practices score, count the number of Best Practices audits, then divide 100 by that number.
Search engine optimization (SEO) is the process of increasing the quality and quantity of website traffic by increasing the visibility of a website or a web page to users of a web search engine.
SEO refers to the improvement of unpaid results (known as "natural" or "organic" results), and excludes direct traffic/visitors and the purchase of paid placement. Primarily SEO pertains to search engine.
First Contentful Paint (FCP) measures the time from navigation to the time when the browser renders the first bit of content from the DOM. This is an important milestone for users because it provides feedback that the page is actually loading.
To speed up First Contentful Paint, speed up the download time of resources or do less work that blocks the browser from rendering DOM content.
- Use HTTP Caching to speed up repeat visits.
- Minify and compress text-based assets to speed up their download time. See Optimizing Encoding and Transfer Size of Text-Based Assets.
Page load is a key aspect of how a user perceives the performance of your page. See Measure Performance with the RAIL Method for more information.
This audit identifies the time at which the user feels that the primary content of the page is visible.
The lower your First Meaningful Paint score, the faster that the page appears to display its primary content.
Optimizing the Critical Rendering Path is particularly helpful towards achieving a faster First Meaningful Paint.
Speed Index is a page load performance metric that shows you how quickly the contents of a page are visibly populated. The lower the score, the better.
To lower your Speed Index score, you need to optimize your page to visually load faster. Two good starting places are:
The First CPU Idle metric measures when a page is minimally interactive:
- Most, but maybe not all, UI elements on the screen are interactive.
- The page responds, on average, to most user input in a reasonable amount of time.
See also Consistently Interactive.
There are two general strategies for improving load time:
- Minimize the number of required or "critical" resources that must be downloaded or executed before the page can load. See Critical Rendering Path.
- Minimize the size of each critical resource. See Optimizing Content Efficiency.
The Time to Interactive (TTI) metric measures how long it takes a page to become interactive. "Interactive" is defined as the point where:
- The page has displayed useful content, which is measured with First Contentful Paint.
- Event handlers are registered for most visible page elements.
- The page responds to user interactions within 50 milliseconds.
Some sites optimize content visibility at the expense of interactivity. This can create a frustrating user experience. The site appears to be ready, but when the user tries to interact with it, nothing happens.