Real User Monitoring: LUX


Our mission is to help designers and developers create joyous, fast user experiences. To do that, we created a real user monitoring (RUM) solution that shows you how your users actually experience the speed of your site.

LUX (which stands for Live User eXperience) gives you standard RUM metrics like page load time and start render. That’s just a start. LUX also tracks metrics that give you a better sense of how fast your site feels to visitors such as number of critical blocking resources, images above the fold, and custom metrics.

Addy Osmani

“Love everything SpeedCurve have been doing lately: First Input Delay support, Long Tasks, CPU Scripting time, Perf Budget support, Last Painted Hero. Y'all are rocking the performance monitoring space big time.”

LUX does even more than track metrics. It helps you discover which design and development improvements will make your site faster, your users happier, and your business more successful.


(LUX demo data comes from Beer Labelizer)

Correlate performance with user experience VIEW DEMO

User engagement charts let you use your own data to understand the impact of page slowdowns on your site's bounce rate.

Correlate performance

Track the impact of performance on business metrics

How do page slowdowns affect your business? Track any metrics you want (including cart size, A/B tests, and conversion rate) using the LUX.addData API.

A/B Testing

Measure what matters most on your pages VIEW DEMO

Custom Metrics

Not all page elements are created equal. Chances are, you want to know how quickly critical content – like product images, call-to-action buttons, or ads – renders on your pages. Using custom timers (via the W3C User Timing spec), you can mark the important times during page load and those custom metrics will be displayed in your LUX dashboard.

Learn when users begin to interact with your pages VIEW DEMO


When users come to your page, you want to know they’re actually engaging with it. This is where Interaction Times come in handy. Interaction Times measure the first moment that a user either scrolls, clicks, or presses a key on a page. These metrics are a good out-of-the-box measure of user engagement (and LUX is the only RUM solution that tracks them).

See the impact of design changes on performance VIEW DEMO

Your LUX Design dashboard lets you see how performance and design affect the user experience. For example:

  • Compare the number of images above-the-fold with the total number of images. This lets you see if you should be lazy loading images outside of the initial viewport.
  • Similarly, comparing viewport size to document size indicates if you have content below-the-fold that you should lazy load.
  • If your start render time is slow, it's good to see how many critical blocking stylesheets and scripts are being served to real users. This is especially important if you have third party tags.

Correlate performance

Create and share custom charts and dashboards

You know your site better than we do. By building custom charts, you can track the metrics that are most important to you. There’s no limit to the number of charts you can build, and the number of separate dashboards you can house them in. Which brings us to...

Tailor dashboards to different audiences

Your technical team wants charts that help them go deep with performance issues. Your executive team wants high-level charts that focus on the metrics they care about. LUX lets you tailor different dashboards for different stakeholders so that people only see the charts and data they need to see.

Create better synthetic tests VIEW DEMO

One of the great things about doing both synthetic monitoring and RUM within SpeedCurve is that you can mash up your data to get better insights. For example, LUX gives you valuable insight into your most-viewed pages, as well as the browsers and viewports that your visitors use. You can use this insight to modify your synthetic test settings to create more accurate, meaningful performance tests.

LUX & Synthetic


(LUX demo data comes from Beer Labelizer)

Getting Started

To get started with LUX, follow the instructions in our LUX Quick Start Guide. It explains how to do everything from inserting the lux.js script in your pages to creating custom metrics and charts.


Our Support Center contains answers to many (and hopefully all!) of your LUX questions. If you can’t find an answer there, email us at

Here’s a handful of the most common questions we’re asked: