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. What rendering metrics 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.
At SpeedCurve, we focus on metrics that capture the user experience. A big part of the user experience is when content actually appears in front of the user. Since stylesheets and synchronous scripts are the culprits when it comes to blocking rendering, we've rolled out some new metrics that focus on these critical blocking resources.
The most helpful innovation we made is to highlight the critical blocking stylesheets and synchronous scripts in our waterfall charts. In the following example waterfall chart for ESPN, notice how the critical stylesheets (green) and synchronous scripts (orange) have a red hash pattern. Not surprisingly, the Start Render metric is delayed until after the last of these critical blocking resources is done loading. The "scrubber" at the bottom of the waterfall (showing the screenshot at that point in time) confirms that rendering has been blocked up to this point. Explore an example of a interactive waterfall chart.
Performance budgets are an important tool for ensuring your site is delivering a great user experience. Steve first experienced performance budgets while Head Performance Engineer at Google. The practice of using budgets to track performance took off with Tim Kadlec's blog post Setting a Performance Budget. The idea is to identify your performance goals and track the metrics that help you achieve your goals.
At SpeedCurve, we give performance budgets first-class status by tracking them in the Site dashboard. Here's an example of tracking a budget for image size.
Before setting your performance budgets, you first have to be monitoring your user experience. Only then can you set budgets and thresholds for improving your baseline user experience. This also allows you to quantify the improvements you're making and share success stories across the organization like "We just improved start render by 32% by reducing image requests to half the budgeted amount".