What is Page Speed?

Broadly speaking, page speed, or load time, refers to the time it takes to download and display the entire content of a web page in the browser, typically measured in seconds. Page speed should also not be confused with site speed. The former refers to an individual page, whereas the latter refers to a sample of pages on a site.

No metric can fully capture the experience of page speed because the loading of a page is not a single moment in time. There are multiple moments (and corresponding metrics) in the loading process that influence user perceptions. In later sections of this page, we'll talk about different metrics that measure various aspects of page load. Because these metrics only measure a moment in the overall load of a web page, these are typically measured in milliseconds.

Understanding the unique insights offered by each of these page speed metrics is important because it allows you to optimize for the ones most important to your site’s user experience.

Calculating Page Speed

Historically, it's been difficult for webmasters to measure page speed. Most metrics don't fully reflect the user experience.

There is also the issue between lab data and field data. Lab data is website performance data collected in a controlled environment with predefined device and network settings. Field data is website performance data collected from real page loads experienced by actual users. With lab data, you are able to understand conditions on the most typical devices and network settings. With field data, you capture true real-world user experience and enable correlation to business KPIs, but there is a restricted set of metrics to work with.

Metrics that attempt to measure page speed include:

  • The Load Event, which measures when the whole page has loaded.
  • The First Paint, when the web browser renders anything different from what was previously on the screen.
  • The First Contentful Paint, which measures when the first bit of content is rendered.
  • The First Meaningful Paint, which measures when the essential content is rendered.
  • The Time To Interactive, which measures the time between when the First Contentful Paint has occurred and when the page reacts to user interactions.
  • The DOM ContentLoaded, which measures when the initial HTML has been fully loaded and parsed.

None of these tell the full story in isolation, nor are they reliable on a large scale. DOMContentLoaded, for example, doesn't correspond with what the user sees on the screen and First Contentful Paint, as another example, only captures the beginning of the loading experience.

Because page speed isn't only about serving content either. It's about providing both a visually complete and usable site. Focusing too much on the visuals and neglecting usability can ruin the user experience. That’s why you need to address page speed from the load time and interactivity angle and consider a “concert” of metrics. The interval between having a visually complete page and a usable one can make or break the user experience.

Here are the key metrics for loading speed:

  • First Paint, which measures how long it takes the browser to render anything on page.
  • First Contentful Paint, which measures how long it takes the browser to visualize the first piece of the DOM model on page.
  • First Input Delay, which measures the time it takes the browser to respond to the first user interaction on page.
  • Largest Contentful Paint, which measures how long it takes for the largest content in the initial viewport — content above the fold — to load.

Here are the key metrics for interactivity and usability:

  • Time To Interactive, which measures the time it takes the page to become fully interactive after the user arrives. Ideally, when the page displays useful content, the page responds to user interactions within 50ms, and event handlers are registered for the most visible page elements.
  • Total Blocking Time, which measures the amount of time during which longer tasks block the main thread, or how long a page is unresponsive until it becomes fully responsive. This metric shows how unresponsive a page is before it becomes fully interactive and how severely page interactivity is affected by long tasks.
  • Cumulative Layout Shift, which measures how much an unexpected layout shift affects the user experience on a page. Think about when content moves around without user input or prior notification, like when an ad inserts itself over a button you were trying to click.

Factors that impact page speed

In order to understand the impact of page speed, let’s review how page load time works:

  • A user enters a URL, submits form, or clicks on a link.
  • The browser makes a request to the server through the network.
  • A request is processed by the server.
  • The server sends a response back to the browser.
  • The browser starts to receive the requested page (time to first byte).
  • The browser parses, loads, renders page content.
  • The entire page becomes available.

There are many factors at play when considering page speed. Some of these include:

  • Hosting server
  • Amount of bandwidth in transit
  • Web page design
  • Number, type, and weight of on-page elements
  • Location
  • Device
  • Browser type
  • Images

Page Speed Standards

Page speed standards matter for at least two reasons.

First, users are impatient and have increasingly higher standards about site performance. They typically won’t wait around for more than a few seconds for a page to load and become interactive. And if it doesn’t load or if it isn’t very responsive, they won’t just bounce for that particular session. Oftentimes, they are gone for good - you won’t get a second chance to impress them. Research has shown that the stress response to delays in mobile speed, for example, is greater than waiting in the checkout line at a physical store. Understand that page speed is a foundational aspect of a good user experience. It’s typically the thing a lot of people “feel” about your online brand. Speed often determines whether customers can find your brand, trust your website, and buy from you.

Second, Google is struggling to keep up with an internet that's quickly expanding in size and complexity. They have a limited amount of resources to fulfill their mission of “organizing the world’s information in an accessible and useful way”. That means their time is fleeting on your website. They have a crawl budget. If you want to make the most of it, if you want more pages crawled and indexed, you need to architect a site that loads quickly.

How fast does a web page need to be?

  • A fast web page loads in under 500 milliseconds
  • An average web page loads between 500 and 1000 milliseconds
  • A slow web page loads between 1 and 2 seconds.

Beyond 2 seconds is extremely slow and will significantly influence page performance. It will also deter users from using the page or getting any information from that page.

Speed Metrics