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Key Speed Metrics: Metrics to Use When Measuring Speed

By Matthew Edgar · Last Updated: December 08, 2023

The first step to addressing your website’s speed is knowing how long it takes your website to load. Many metrics can be used to evaluate your website’s load time. However, you do not need to track each of these metrics. Each metric matters only in specific circumstances so you can focus on a limited set of metrics and still make progress in improving your website’s performance.

In this article, I’ll review the key speed metrics you can use on your website, including Google’s Core Web Vitals metrics. For more information about each of these metrics and ways to measure these metrics, check out my book Speed Metrics Guide.

Basic Speed Metrics

There are a few basic speed metrics that should be measured for every website. These metrics help you understand important steps within the website loading process.

  • Time to First Byte (TTFB). TTFB measures how long it takes from when a URL is requested to when the first byte of information is returned from the server. If TTFB is high, that indicates problems with your website server and hosting configuration. If TTFB is slow, all other metrics will be slow as well.
  • First Contentful Paint (FCP). FCP measures how long it takes between the request for a URL and when content is painted, or displayed, on the page for visitors. If this metric is high, that indicates your website is loading too many files (images, JavaScript, CSS, etc.) and those files are delaying the browser’s ability to paint the webpage.
  • Transfer Size. Transfer size measures how many bytes of data need to be sent from the server to the browser to load the webpage. You can look at the transfer size overall (for all files loaded) or total bytes for specific files.
  • Time to Interactive (TTI). TTI measures when visitors can begin reliably interacting with the webpage. Before TTI, if visitors tried to click on links or interact with any features on the page, the page might be slow to respond. TTI is usually slowed by an excessive amount of JavaScript on the page. If TTI is slow, then Google’s Core Web Vitals will usually be slow as well.
  • Fully Loaded Time. This metric tells you how long it takes between the request of the URL and when everything has finished loading into the browser. Typically, visitors can interact with the website and see the page before Fully Loaded Time. However, Fully Loaded Time is an indication of when the website is done loading the page.

Google’s Core Web Vitals

Google measures speed by evaluating how quickly real visitors have loaded the website in Google Chrome over the last 28 days. The metrics evaluated are referred to as Core Web Vitals and they can influence where a website ranks in search results. The two metrics evaluated as part of Core Web Vitals that relate the most to speed are:

  • Interaction to Next Paint (INP). INP is a pending Core Web Vitals metric. Google will replace First Input Delay (FID) with INP in March of 2024. INP measures the time in between a user interacting with a webpage (like clicking a button or a link) and the result of that interaction appearing on the user’s screen. For Core Web Vitals, Google will observe all interactions with a webpage but will only take the longest INP observed into consideration.
  • Largest Contentful Paint (LCP). LCP measures when the largest element on any page of your website is rendered (displayed) in the browser. LCP is not a measure of total load time. Instead, LCP measures how long a visitor to your website must wait before they can see the main content of the page above the first scroll on their screen. The longer people must wait the worse the experience.

Google also includes Cumulative Layout Shift (CLS) as a Core Web Vital metric. While related to speed, CLS is a measure of visual shifting and not a precise measure of website speed. You can learn more about CLS and the other Core Web Vitals metrics in my article about Google’s Core Website Vitals.

How Fast Should My Website Load?

As you begin looking at different metrics, the question becomes how fast should your website load. What are the benchmarks? What does Google require for Core Web Vitals?

Basic Speed Metrics Guidelines

Google provides specific guidelines for some of the basic speed metrics for these to be considered fast:

  • TTFB: below 800 milliseconds and anything over 1.8 seconds is considered slow (source)
  • FCP: below 1.8 seconds and anything over 3 seconds is considered slow (source)
  • TTI: below 3.8 seconds and anything over 7.3 seconds is considered slow (source)

There are no clear benchmarks for transfer size other than keeping it as small as possible. Google limits its crawl to the first 15MB of each requested file (though most websites fall well below this level). According to HTTP Archive, the total transfer size for all requested files is 2.4 megabytes on desktop websites and 2.2 megabytes on mobile websites.

There are also no clear benchmarks for Fully Loaded Time. A similar metric of onload time (onload comes before Fully Loaded Time) does have some reliable industry averages. According to HTTP Archive, desktop websites have an onload time of 6.5 seconds and mobile websites have an onload time of 10.6 seconds. At Elementive, we recommend websites should stay under these averages as best as possible.

All of these are recommendations. Plenty of successful websites do not load their websites this quickly. With all of these metrics, it depends on exactly how the website’s speed is affecting visitors as to whether or not load times greater than these recommendations really present a critical problem for your website. As well, the more competitive your search rankings, the faster all of these metrics should be.

Core Web Vitals Guidelines

Core Web Vitals metrics do have specific thresholds.

  • Interaction to Next Paint (INP). The longest INP observed should be under 200 milliseconds to be considered fast and a response over 500 milliseconds will be considered slow. (source) Only click, tap, and key press interactions are observed. Scrolling and hovering interactions are not observed (currently). You can improve INP by ensuring the server responds quickly (which will improve TTFB) and ensuring that the code is optimized to respond quickly to a visitor’s interactions.
  • Largest Contentful Paint (LCP). The largest content element on the page should load in 2.5 seconds or less and anything greater than 4 seconds will be considered slow. (source) If you size images appropriately, reduce your JavaScript and CSS as much as possible, reduce third-party scripts, and other similar actions, that should help improve LCP. Improving LCP also means you are improving your website’s total load time as well.

If the website exceeds these thresholds, it can negatively influence your search rankings. However, it is important to remember that Core Web Vitals is not the most important ranking factor. Instead, it operates as a ranking boost and applies if the website already ranks in search results. This means Google will not rank a website highly in results simply because it loads fast. As well, this means Google will not remove an otherwise helpful and high-quality website from search results because it loads slowly. That said, it is still important to stay as close to these requirements as possible.

Google Crawl Speed: Average Response Time

Another important metric to monitor for SEO is how quickly Google can load the website during a crawl, measured by Average Response Time. This can be found in Google Search Console’s Crawl Stats report. To access this report, click on Settings in the Sidebar and then click Open Report under the Crawling section. This will show the Average response time in milliseconds. This is a measurement of how quickly Google retrieves the page’s content. It excludes the time to render the content (execute JavaScript) and excludes the time to load dependent files (images, JavaScript files, iframes, and so on). Google does not have any specific guidelines around this but generally, the response time should be kept below 1,000 milliseconds (one second). Also, the response time should be steady over time, and any spikes, like those seen in the following screenshot, could indicate new problems and should be investigated.

Crawl Stats Report showing Average response time

Final Thoughts

There are many metrics describing how quickly a website loads. It is important to select the metrics that are the most meaningful for your website and continually evaluate those to identify opportunities to improve speed on your website.

For more information, check out the following articles or if you need help measuring and optimizing speed on your website, contact me today.

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