Posts tagged with
" CPU"

Measuring Jank and UX

Ten years ago the network was the biggest problem when it came to making websites fast. Today, CPU is the main concern. This happened because networks got faster while JavaScript moved in the other direction growing 3x in size in the last six years. This growth is important because JavaScript consumes more CPU than all other browser activities combined. While JavaScript and other activities block the CPU, the browser can't respond to user input creating the sensation of a slow, jittery, or broken page, AKA "jank".

To help focus our attention on CPU, several new performance metrics have been defined and evangelized over the last year or three. In this post I'm going to focus on these:

  • First CPU Idle measures when the page is no longer janky. Specifically, it is the first span of 5 seconds where the browser main thread is never blocked for more than 50ms after First Contentful Paint. A value of 2-4 seconds is typical.
  • First Input Delay measures the gap between when a user interacts with the page (e.g, clicks or scrolls) and when the browser is able to act on that input. First Input Delay values are much lower - a good target is 10ms, but 25ms is common.
  • First Interaction Time is when the first user input takes place. This varies widely depending on the type of site and page. A good search results page might have a low First Interaction Time because users scroll and click quickly. A media site might have a high First Interaction Time because users start reading content (headlines, stories) before interacting with the page. At SpeedCurve we call this "IX Time".
  • Total Long Task CPU Time is the sum of all long tasks that occur in the page. A "long task" is a browser event that blocks the main thread for more than 50ms.

Here's a figure to help visualize these metrics.

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New LUX JavaScript Dashboards

As organizations work to improve performance for users around the world on slower networks and devices, the focus on JavaScript continues to grow. LUX's new JavaScript dashboards help to identify the problems and solutions for creating a fast, joyous user experience.

LUX is SpeedCurve's real user monitoring product. We launched it two years ago with four dashboards: Live, Users, Performance, and Design. Today we've added two more LUX dashboards: JavaScript and JS Errors. These new dashboards let you see the impact JavaScript has on your site and on your users with new metrics, including First CPU Idle and First Input Delay, and new features, such as correlation charts that show you how CPU time correlates with bounce rate.

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JavaScript dominates browser CPU

Loading scripts asynchronously is critical for getting pages to render more quickly. We care about rendering because that's what users see; if rendering is slow users have a negative experience. But it's not just about what users see - how the site feels is also important. That's why we focus so much on CPU time. If the CPU is blocked, then browsers are delayed responding to user interactions like scrolling and clicking on links. In other words, the page feels janky. And what consumes the most CPU in browsers? You guessed it: JavaScript!

Desktop median CPU times

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New LUX metrics

Over the winter holiday we added a bunch of new metrics to LUX:

  • First Contentful Paint
  • First CPU Idle
  • Longest Long Task
  • Number of Long Tasks
  • Connection type
  • HTML transfer size
  • Total # of image requests

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First Input Delay shows how quickly your site responds to user interaction

We're excited to announce the availability of the First Input Delay metric as part of LUX, SpeedCurve's RUM product.

First Input Delay

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