"CPU"
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.
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!
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
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.
Using RUM to track CPU time
It's exciting working at SpeedCurve and pushing the envelope on performance monitoring to better measure the user's experience. We believe when it comes to web performance it's important to measure what the user sees and experiences when they interact with your site. A big part of our focus on metrics has been around rendering including comparing TTI to FMP, Hero Rendering, and critical blocking resources.
The main bottleneck when it comes to rendering is the browser main thread getting blocked. This is why we launched CPU charts for synthetic testing over a year ago. Back then it wasn't possible to gather CPU information using real user monitoring (RUM), but the Long Tasks API changes that. Starting today, you can track how CPU impacts your users with SpeedCurve's RUM product, LUX.
Track down front-end CPU hogs
Often when monitoring and debugging site performance we focus on network activity and individual resources, but what about the CPU? As more and more sites switch to using large Javascript frameworks and manipulating the page using Javascript, the execution time this code takes and the available CPU can instead become the performance bottleneck.
CPU usage for all Chrome tests
For any of SpeedCurve's Chrome-based tests, including emulated devices, we capture the Chrome Dev Tools timeline. From the browser main thread usage in the timeline we extract how busy the CPU is and what it's spending time on. Is it busy executing Javascript functions or busy laying out elements and painting pixels?
We also measure the CPU usage to different key events in the rendering of the page. SpeedCurve's focus is on the user experience and getting content in front of people as fast as possible, so we show you what the CPU is doing up till the page starts to render. This reflects CPU usage during the browser critical rendering path and can highlight various issues. If there's lots of CPU idle time then you're not delivering your resources efficiently. You want to get the CPU busy nice and early rendering the page, rather than sitting idle waiting for slow resources.
In the test below we see in the first pie chart that the CPU is spending a lot of time on layout up to the start render event, which is quite a different picture from the Fully Loaded CPU usage.
