What is Lighthouse and How Do I Make Sense of the Metrics?

Loading Speed Clock Computer Screen

What is Lighthouse and How Do I Make Sense of the Metrics?

Google’s Lighthouse is a program that allows users to test the speed, loading times, and optimization of a website.  It provides important information if a user is interested in optimizing the speed with which their website loads. If you are interested in improving the performance of your website, it’s one of the first places you should check if you are looking for areas you can improve in.  Lighthouse tracks a variety of metrics, but there are a handful that are the most important to keep track of. Learn more about a few of the key metrics by reading below:

Time To Interactive

This metric is just as it sounds: it measures how long it takes for a page to become fully interactive.  Lighthouse defines an interactive page as page that has displayed it’s useful content and responds to user requests within 50 milliseconds.

Speed Index

Speed index is another fairly self-explanatory metric.  It measures how long it takes for the contents of a page to become visibly populated, meaning when they become visible to the user. Ideally you want a time under 3 seconds, but the maximum possible score is awarded if your page loads in a shade under two seconds.

First Contentful Paint

First Contentful Paint is a slightly more confusing metric. Essentially, what it measures is the time from when you first attempt to open a page or conduct a search, to the point where it becomes obvious that the page is loading.  Think about when you Google something, and even though the whole search engine result page hasn’t loaded yet, the search bar at the top of the page has become visible.  It is evident to you that your page is loading, but you cannot see the content that you are searching for yet.  First Contentful Paint is more or less the moment where it becomes clear that your request to load a page is occurring, but you can’t see the content yet.

First CPU Idle

First CPU Idle, which was previously called Time to First Interactive, measures how long it takes for a page to become at least partially interactive within a reasonable amount of time.  It’s a measure of when most of the interactive elements on a page (like menus) are utilizable by users, even though the page is still loading.  This is probably the most complicated metric to quantify, as it varies from site to site, and even page to page, depending on how long they take to load.

First Meaningful Paint

This metric is the meat and potatoes of what Lighthouse measures.  FMP measures how long it takes for a pages primary content to be displayed on screen.  Primary content varies by page. For a search engine, primary content would be the results themselves.  For a blog article it would be the headlines and text.  No matter what the primary content on your page is, this is the most important metric in terms of SEO. Having a site that loads quickly and provides users with the information they are looking for keeps users happy and on your website.

Understanding the metrics that Lighthouse measures can help you learn more about how you can optimize your websites load speeds, keeping users and Google happy and increasing your websites authority!

John Wagenkenecht
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