Bounce Rate
Bounce rate is defined as the percentage of visitors to a website who leave after viewing only one page. If your webpage has a low PageSpeed score, the probability of a user leaving your site can increase by over 120%.
The Metrics used for Judging a Good Page Score
First Contentful Paint
First contentful paint marks the time at which the first text or image is painted. An FCP score is a comparison between your page’s FCP time and the FCP times for other real websites.
| FCP Time (seconds) | Ranking |
| 0-1.8 | Green (Fast) |
| 1.8-3 | Orange (Moderate) |
| Over 3 | Red (Slow) |
Time To Interactive
Time to interactive is the amount of time it takes for the page to become fully interactive. A page is considered fully interactive when:
- The page displays useful content, measured by first contentful paint
- Event handlers are registered for most visible page elements
- The page responds to user interactions within 50 milliseconds.
| TTI Metric (seconds) | Ranking |
| 0-3.8 | Green (Fast) |
| 3.8-7.3 | Orange (Moderate) |
| Over 7.3 | Red (Slow) |
Speed Index
Speed index shows how quickly the contents of a page are visibly populated. A speed index score is a comparison between your page and other real websites.
| Speed Index (seconds) | Ranking |
| 0-3.4 | Green (Fast) |
| 3.4-5.8 | Orange (Moderate) |
| Over 5.8 | Red (Slow) |
Total Blocking Time
Total blocking time (TBT) measures the total time between your page’s first contentful paint and time to interactive, where the main thread was blocked for long enough to prevent input responsiveness.
The main thread is considered “blocked” any time there’s a task that runs on the main thread for more than 50ms. If the task is long enough, users will likely notice the delay and the page will appear sluggish and janky.
A good TBT score is generally considered to be less than 200ms.
Largest Contentful Paint
Largest contentful paint (LCP) measures the time it takes for the largest image or block of text to render on the page. Elements that are considered for LCP are images, videos, text blocks and background images.
| LCP Time (seconds) | Ranking |
| 0-2.5 | Green (Fast) |
| 2.5-4 | Orange (Moderate) |
| Over 4 | Red (Slow) |
Cumulative Layout Shift
Cumulative layout shift (CLS) is used to measure stability of your page and helps to quantify how often the page unexpectedly shifts. This includes things like text moving whilst you’re reading an article or links and buttons moving when you click on them.
| CLS Time (seconds) | Ranking |
| 0-0.1 | Green (Fast) |
| 0.1-0.25 | Orange (Moderate) |
| Over 0.25 | Red (Slow) |
