Google PageSpeed Insights gets its lab data from Lighthouse, which is an open-source tool. This tool determines how good the website’s performance is by crawling the URL.
Here, we once again find the categories of “fast”, “average”, and “slow”, as well as the FCP, LCP, and CLS factors. However, these values have different ranges in the lab data. Two further values are also provided here: Time to Interactive (TTI), which is the total time it takes for a page to become interactive, after its Total Blocking Time (TBT), the total time during which it was unresponsive.
The sixth metric given in the lab data is the Speed Index. The speed index measures how quickly the content is displayed visually as the website loads. Lighthouse first takes a video of the page as it loads in the browser and then evaluates the visible progression between the frames.
The evaluation benchmarks used for the lab data are as follows:
Fast (green):
- FCP: between 0 and 2 seconds.
- LCP: between 0 and 2 seconds.
- CLS: the shift is between 0 and 10%.
- TTI: between 0 and 3.8 seconds.
- TBT: between 0 and 0.3 seconds.
- Speed index: between 0 and 4.3 seconds.
Average (orange):
- FCP: between 2 and 4 seconds.
- LCP: between 2 and 4 seconds.
- CLS: the shift is between 10 and 25%.
- TTI: between 3.9 and 7.3 seconds.
- TBT: between 0.3 and 0.6 seconds.
- Speed index: between 4.4 and 5.8 seconds.
Slow (red):
- FCP: more than 4 seconds.
- LCP: more than 4 seconds.
- CLS: the shift is more than 25%.
- TTI: more than 7.3 seconds.
- TBT: more than 0.6 seconds.
- Speed index: more than 5.8 seconds.
One of Lighthouse’s practical features is how it displays screenshots including time data, particularly when analyzing the cumulative layout shift (CLS), to show how the page loads in.