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:
Here's a figure to help visualize these metrics.
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We're excited to announce that we've launched Last Painted Hero as an official metric. Last Painted Hero is a synthetic metric that shows you when the last piece of critical content is painted. Keep reading to learn how Last Painted Hero works, why (and how) we created it, and how it can help you understand how your users perceive the speed of your pages.
When choosing the right performance metric, my soapbox for the last few years has been "not every pixel has the same value". In other words, rather than chase dozens of different performance metrics, focus on the metrics that measure what's critical in your page.
Here at SpeedCurve, we think it's good to focus on rendering metrics, because they're a closer approximation to what the user experiences. There are some good rendering metrics out there, like start render and Speed Index, but the downside to these metrics is that they give every pixel the same value. For example, if the background renders and some ads render, that could improve your start render time and Speed Index score, but it might not have a big impact on the user's experience. Instead, it's better to measure the parts of the page that matter the most to users. We call those parts of the page the "hero elements".
At SpeedCurve, we're fond of the phrase "a joyous user experience". Creating this joy requires delivering what users want as quickly as possible. It's important that the critical content is downloaded and rendered before users get frustrated.
Network metrics have been around for decades, but rendering metrics are newer. Speed Index. Start Render. Time to First Interactive. First Meaningful Paint. These are a few of the rendering metrics that currently exist. What do they mean? How do they compare? Which are best for you? Let's take a look.
The key to a good user experience is quickly delivering the content your visitors care about the most. This is easy to say, but tricky to do. Every site has unique content and user engagement goals, which is why measuring how fast critical content renders has historically been a challenging task.
That's why we're very excited to introduce Hero Rendering Times, a set of new metrics for measuring the user experience. Hero Times measure when a page's most important content finishes rendering in the browser. These metrics are available right now to SpeedCurve users.
More on how Hero Rendering Times work further down in this post. But first, I want to give a bit of back story that explains how we got to here.
SpeedCurve reports the number of critical blocking resources in the page. These are the resources that block rendering. Since it's important that users see your content as quickly as possible, it's important to know what might be causing your page to render slowly. We recently enhanced the way we measure blocking resources and wanted to share those improvements with our customers as well as the performance community at large.
The main culprits that block rendering are scripts and stylesheets that are loaded synchronously. A great way to avoid this blocking problem is to load your scripts and stylesheets asynchronously. You can do that for scripts by using the async and defer attributes, plus other programmatic techniques. Loading stylesheets asynchronously is less popular but is still possible using techniques like loadCSS.
SpeedCurve’s sweet spot is the intersection of design and performance - where the user experience lives. Other monitoring services focus on network behavior and the mechanics of the browser. Yet users rarely complain that “the DNS lookups are too slow” or “the load event fired late”. Instead, users get frustrated when they have to wait for the content they care about to appear on the screen.
The key to a good user experience is quickly delivering the critical content.