October 12, 2018
Time is perceived subjectively by all of us therefore, optimizing for what directly affects this perception should be an essential part of every performance strategy – always prioritize the user experience.
Today I'd like to introduce you to Time to Interactive – a performance metric that focuses on what users see and experience.
How can metrics give us an understanding of what users actually experience?
Historically, page load time was used as an indicator of performance, but ultimately it did not in any way describe a user experience other than the page had fully loaded.
There is a trend of people recommending and using paint based metrics as part of their web performance analysis. First Paint and First Meaningful Paint are very useful indicators because they describe key moments of our content rendering.
Page load time and paint based metrics do not tell us enough.
You’ve likely once tapped on a link, at the moment when the browser decided to render more content, change the font or some other jarring moment. Suddenly, without any other input from your side, you’ve been relocated to a different section on the page or even another website. Frustrating, right? The reason behind this behavior is a non-interactive moment when the browser’s main thread is blocked. 🙈
For a better understanding of this metric, it's valuable to explain first what is the main thread and then why it can be locked.
Seeing as all user interface interactions and updates are managed in this single-threaded environment, the main thread becomes blocked when a single long-running operation prevents the completion of the next process in the pipeline.
Subtly blocking the main thread is far more destructive on mobile devices. Tasks that take 50ms on a high powered desktop can take 3-5X longer on a phone. That results in lockups, poor animation, janky scrolling and battery drain.
First Interactive and Consistently Interactive, later known as First Idle and Time to Interactive, are defined and calculated as follows:
These metrics help us to accurately calculate when an application is ready for interaction, and will likely respond in timely manner.
Optimizing your web app or site with Time to Interactive in mind means better user experience since you’ll mitigate the risk of blocked UI elements.
In a recent case study about Pinterest’s Progressive Web App, the team observed 60% higher user engagement after reducing Time to Interactive from 23s to 5.6s, and to 3.9s on repeat visits! 👏
First Idle and Time to Interactive can be monitored using Calibre’s automated web performance tests.
WebPageTest shows when the main thread is responsive as part of the waterfall diagram.
Time to Interactive is also integrated into Lighthouse. You can run the performance audit report from your Chrome browser dev tools.
Time to interactive gives us valuable insight to people’s experience when using our web apps. The trend of creating more human-centric performance metrics is the right direction. It helps us to understand the importance of creating accessible and responsive products.
I hope you enjoyed this introduction. Let me know if you have any questions!
Radimir is a Frontend engineer at Project A Services. Find him on Twitter.
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