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“How fast is a website?” While it’s a simple question on the surface, it can be challenging to answer. In the past, the answer would be based on the load time of a page, but over the years, we have evolved our approach to site speed to incorporate new metrics, alone or in combination with existing metrics. To build fast sites and stay competitive, it is critical for people passionate about performance to stay informed about new metrics and the methodology behind them. Today, when asking “how fast,” we need to look at more than load speed and paint metrics and incorporate interactivity and user experience.

The Evolving View of Performance

Originally, web performance was quantified with metrics around when certain content was loaded, with events such as DOM Complete or Onload. While these are helpful, they primarily measure the browser’s experience, as opposed to what the user experiences.

The next evolution of performance brought in paint metrics. These metrics, which include Start Render, Visually Complete, First Contentful Paint and First Meaningful Paint, measure the user’s actual experience more directly by tracking different points in time when content is drawn inside the browser. While it might not be obvious to a user when the window.onload JavaScript event fires, it is obvious when the text content first appears because the user can start reading it!

Paint metrics continue to be important, but we have learned that users don’t only want to see what the content on a page looks like but they also want to interact with it. This is where so-called interactivity metrics come in. Interactivity metrics attempt to measure when the browser is capable of responding to user actions. Just because a button image appears on the screen doesn’t mean that it will do something if the user clicks on it.

Modern Interactivity Metrics

Interactivity is a fairly new area of focus, and it can be challenging to define the concept. Metrics like First Interactive and Google’s Consistently Interactive metric were early attempts at quantifying interactivity. However, as an industry, we have adopted newer, more accurate metrics: Time to Interactive and First CPU Idle.

Time to Interactive (TTI) measures the amount of time it takes for an application or site to be “interactive,” defined as the point at which:

  • The page has displayed useful content, which is measured with First Contentful Paint.
  • Event handlers are registered for most visible page elements.
  • The page is able to respond to user interactions within 50 milliseconds.

Imagine that a user takes the time to navigate to your site and is able to see headers and images, but when they try to take an action, the site lags and gives no indication that anything is happening. Out of frustration, the user closes the site and goes to your competitor’s site instead. This may seem trivial, but it happens every day, and businesses lose millions of dollars because of it.

The motivation behind TTI is to provide greater insight into the efficiency of an application while a user is interacting with it. TTI begins with the start of First Contentful Paint and requires that the browser is quiet for at least 5s and that there are no long tasks that will cause jank (a lagging, stiff experience) in response to user interaction.

While the actual mechanics of TTI can be fairly complex (we provide all the nerdy technical details here), the important takeaway is that TTI is the next step in quantifying interactivity and what it means in the scope of performance.

First CPU Idle (FCI) is similar to TTI in that they are both trying to represent similar events, but the way they are calculated is different. It measures when a page is considered minimally interactive, which is defined as the point when:

  • Most, but not all, UI elements on the screen are interactive.
  • The page responds, on average, to most user input in a reasonable amount of time.

So why measure FCI? First, it is used as a factor in the Google Lighthouse Performance Score. Second, your organization or partners might already be using FCI as a metric, so tracking it is important for consistency and making sure that everyone is speaking the same language about metrics.

Why Should You Care About the Latest Performance Metrics?

In the beginning of web performance monitoring, there was a great emphasis on Page Load Time because everything was about speed and how seconds translate to dollars. Then came the Paint Metrics, when many realized that it was not just about the page loading quickly but also about what specific content the user can see. As we now continue into the era of interactivity, we understand that users not only want to see the content loaded on the page and want it to be fast but they also want to interact directly with it. Thus, if you care about web performance and are interested in improving the user experience of your site, you need to track the latest metrics – but how?

Rigor Performance TTI KPI

Rigor, in keeping with the evolving nature of the performance industry, now offers these interactivity metrics through our platform. You can track and trend on our Performance KPI chart, create an interactivity centered data block, or even benchmark yourself with Alexa’s Top 5000 best-performing sites through the Executive Dashboard. Understanding that measuring web performance from multiple angles and adopting new strategies is key to staying ahead. To learn more about the Rigor platform and our interactivity metrics, request a free trial today.

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