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Understanding the performance impact of anti-flicker snippets

Experimentation tools that use asynchronous scripts – such as Google Optimize, Adobe Target, and Visual Web Optimizer –  recommend using an anti-flicker snippet to hide the page until they've finished executing. But this practice comes with some performance measurement pitfalls:

  • Hiding the contents of the page can have a dramatic effect on the Web Vitals that measure visual experience, such as First Contentful Paint (FCP) and Largest Contentful Paint (LCP)
  • Anti-flicker snippets can also affect Cumulative Layout Shift (CLS) and the synthetic equivalent of First Input Delay (FID), Total Blocking Time (TBT).

In this post we'll look at how anti-flicker snippets work, their impact on Web Vitals, and how to measure the delay they add to visitors' experience.

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SEO and web performance: What to measure and how to optimize

Chances are, you're here because of Google's update to its search algorithm, which affects both desktop and mobile, and which includes Core Web Vitals as a ranking factor. You may also be here because you've heard about the most recent potential candidates for addition to Core Web Vitals, which were just announced at Chrome Dev Summit. 

A few things are clear:

  • Core Web Vitals, as a premise, are here to stay for a while.
  • The metrics that comprise Web Vitals are still evolving.
  • These metrics will (I think) always be in a state of evolution. That's a good thing. We need to do our best to stay up to date – not just with which metrics to track, but also with what they measure and why they're important.

If you're new to Core Web Vitals, this is a Google initiative that was launched in early 2020. Web Vitals is (currently) a set of three metrics – Largest Contentful Paint, First Input Delay, and Cumulative Layout Shift – that are intended to measure the loading, interactivity, and visual stability of a page. 

When Google talks, people listen. I talk with a lot of companies and I can attest that, since Web Vitals were announced, they've shot to the top of many people's list of things to care about. But Google's prioritization of page speed in search ranking isn't new, even for mobile. As far back as 2013, Google announced that pages that load slowly on mobile devices would be penalized in mobile search. 

Keep reading to find out:

  • How much does web performance matter when it comes to SEO?
  • Which performance metrics should you focus on for SEO?
  • What can you do to make your pages faster for SEO purposes?
  • What are some of the common issues that can hurt your Web Vitals?
  • How can you track performance for SEO?

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UPDATE: Bookmark and compare synthetic tests

One of the huge benefits of tracking web performance over time is the ability to see trends and compare metrics. Last year we added new functionality that makes it easy for you to bookmark and compare different synthetic tests in your test history. We recently added some additional enhancements to make comparing tests even easier.

With the 'Compare' feature, you can generate side-by-side comparisons that let you not only spot regressions, but easily identify what caused them:

  • Compare the same page at different points in time
  • Compare two versions of the same page – for example, one with ads and one without
  • Understand which metrics got better or worse
  • Identify which common requests got bigger/smaller or slower/faster
  • Spot any new or unique requests – such as JavaScript and images – and see their impact on performance

Along the way, we've also made it much more intuitive for you to drill down into your detailed synthetic test results. Let's take a look...

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NEW: Lighthouse v8 support!

Google Lighthouse logo

After Google's announcement about Lighthouse 8 this past month, we have updated our test agents. We've gotten a lot of questions about what has changed and the impact on your performance metrics, so here's a summary.

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Web Performance for Product Managers

I love conversations about performance, and I'm fortunate enough to have them a lot. The audience varies. A lot of the time it’s a front-end developer or head of engineering, but more and more I’m finding myself in great conversations with product leaders. As great as these discussions can be, I often walk away feeling like there was a better way to streamline the conversation while still conveying my passion for bringing fellow PMs into the world of webperf. I hope this post can serve that purpose and cover a few of the fundamental areas of web performance that I’ve found to be most useful while honing the craft of product management.

So, whether you are a PM or not, if you're new to performance I've put together a few concepts and guidelines you can refer to in order to ramp up quickly. This post covers:

  • What makes a page slow?
  • How is performance measured?
  • What do the different metrics mean?
  • Understanding percentiles and how to use them
  • How to communicate performance to different stakeholders

Let's get started...

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An Opinionated Guide to Performance Budgets

Performance budgets are one of those ideas that everyone gets behind conceptually, but then are challenged to put into practice – and for very good reason. Web pages are unbelievably complex, and there are hundreds of different metrics available to track. If you're just getting started with performance budgets – or if you've been using them for a while and want to validate your work – this post is for you.

What is a performance budget?

A performance budget is a threshold that you apply to the metrics you care about the most. You can then configure your monitoring tools to send you alerts – or even break the build, if you're testing in your staging environment – when your budgets are violated.

Understanding the basic premise of performance budgets is pretty easy. The tricky part comes when you try to put them into practice. This is when you run into three important questions:

  1. Which metrics should you focus on?
  2. What should your budget thresholds be? 
  3. How do you stay on top of your budgets?

Depending on whom you ask, you could get very different answers to these questions. Here are mine.

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New! Vitals Dashboard

Getting up to speed on Core Web Vitals seems to be at the top of everyone's to-do list these days. Just in time for the holidays, we are happy to bring you our new Vitals dashboard to help you get a huge jumpstart.

We love to visualize performance data (in case you haven't heard). We love it even more when we can be true to one of our biggest motivations at SpeedCurve: leveraging both RUM and Synthetic data to help you take action on what matters most.

Vitals Dashboard

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First Input Delay: How vital is it?

We’ve been pretty vocal about Core Web Vitals since Google announced this initiative last spring. We love the idea of having a lean, shared set of metrics that we can all rally around – not to mention having a broader conversation about web performance that includes teams throughout an organization.

For many site owners, the increased focus on Core Web Vitals is driven by the fact that Google will be including them as a factor in search ranking in May 2021. Other folks are more interested in distilling the extremely large barrel of performance metrics into an easily digested trinity of guidelines to follow in order to provide a delightful user experience.

We’ve had some time to evaluate and explore these metrics, and we're committed to transparently discussing their pros and cons.

The purpose of this post is to explore First Input Delay (FID). This metric is unique among the three Web Vitals in that it is can only be measured using real user monitoring (RUM), while the other two (Largest Contentful Paint and Cumulative Layout Shift) can be measured using both RUM and synthetic monitoring.

In this post we'll cover:

  • What is FID?
  • What does FID look like across the web?
  • The importance of measuring user interactions
  • How JavaScript affects user behavior
  • Suggestions for how you can look at FID in relation to your other key metrics

Let's dig in!

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Cumulative Layout Shift: What it measures, when it works (and doesn't), and how to use it

Back in May, we shared that SpeedCurve supports Google's Core Web Vitals in both our synthetic monitoring and real user monitoring tools. Two of the Web Vitals – Largest Contentful Paint (LCP) and First Input Delay (FID) – were actually available in SpeedCurve for quite a while prior to the announcement. The newcomer to the scene was Cumulative Layout Shift (CLS), and, not surprisingly, it's the metric that's gotten the most questions.

A few of the questions I've been asked (or asked myself) about Cumulative Layout Shift:

  • What does CLS measure?
  • How is it calculated?
  • What does it mean in terms of actual user experience?
  • Does it correlate to user behaviour or business metrics in any measurable way?
  • What are the (inevitable) gotchas? 
  • Ultimately, how much should we care about CLS?

Six months in, I've had a chance to gather and look at a lot of data, talk with customers, and learn from our friends in the performance community. Here's what I've learned so far.

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Visualizing Layout Shifts

One of the big challenges with Google's new Cumulative Layout Shift (CLS) metric is understanding which elements actually moved on the page, when they moved, and by how much. To help with debugging your CLS scores, we've added a new visualization to SpeedCurve that shows each layout shift and how each individual shift adds up to the final cumulative metric.

CLS Layout Shift

For each layout shift, we show you the filmstrip frame right before and right after the shift. We then draw a red box around the elements that moved, highlighting exactly which elements caused the shift. The Layout Shift Score for each shift also helps you understand the impact of that shift and how it adds to the cumulative score.

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Fast badging for better UX

Is your site fast? Adding to a string of recent announcements including Lighthouse v6 and Core Web Vitals, Google has introduced Fast page labelling in Chrome 85 for Android. If you are curious about what this means for your site and how you can get in front of it, read on!

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NEW: Lighthouse v6 support!

Google Lighthouse logo

Lighthouse v6 has arrived! The much-anticipated update to Lighthouse is now available to SpeedCurvers as part of our latest test agent updates. Keep reading to find out what this means and how it may affect your performance metrics.

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Tracking Web Vitals for a better user experience

SpeedCurve chart showing FCP, FID and CLS.

Google recently announced an initiative called 'Web Vitals', which focuses on three performance metrics they consider essential for improving the user experience:

  • Largest Contentful Paint (LCP)
  • First Input Delay (FID)
  • Cumulative Layout Shift (CLS)

With the exception of FID, all of these metrics are available in both RUM and Synthetic. FID requires a real user for calculation and therefore is only available in RUM. In place of FID for Synthetic, we recommend tracking JS Long Tasks or Total Blocking Time as an alternative CPU metric.

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User Timing Level 3: Set a value!

The Chrome team has snuck a fundamental change to how user timing marks work into Chrome 78. For many years now, our own Steve Souders has championed user timing, and he has always believed you should be able to set a value for a user timing mark or measure.

With User Timing Level 3, now you can! Level 3 lets you explicit set a startTime whereas previously you could only pass in the name of a mark.

performance.mark("name", {startTime: 1000})

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Introducing Page Speed Benchmarks - a new resource for the performance community


Here are some common questions I’m asked when I talk with people about performance:

  • Which metrics should I care about?
  • What types of devices and connections should I test on?
  • Which third parties should I be most concerned about?
  • How fast should I be?
  • What are some good sites I can use for benchmarking?

Today, I’m very excited to announce the release of a new project that helps answer those questions – and more! 

Page Speed Benchmarks is an interactive dashboard that lets you explore and compare web performance data for leading websites across several industries – from retail to media.

With Page Speed Benchmarks, you can do things like:

  • See what the different metrics actually mean in terms of user-perceived performance
  • Compare how the same page renders on fast vs slow devices and connections
  • Understand what makes fast sites fast (and slow sites slow)
  • Get insights into how third parties can perform on different sites
  • Identify sites you can use for your own competitive benchmarking

If you already like tools like the HTTP Archive, I think you'll love how you can use Page Speed Benchmarks to complement the insights you're already getting. Keep reading to find out how we set up these benchmarks, and how you can mine our test data – even if you're not a SpeedCurve user – for your own performance research.

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Six web performance resolutions for the new year

For the past two years, the performance.now() conference has been the most valuable performance event I've attended. So valuable, in fact, that I've made some of the talks the cornerstone of this list of performance resolutions for 2020. I'd love to know how many – if any – of these are on your list. As always, I'd love people's feedback!

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New! User Happiness metric, CI plugin, and an inspiring third-party success story

Here at SpeedCurve, the past few months have found us obsessing over how to define and measure user happiness. We've also been scrutinizing JS performance, particularly as it applies to third parties. And as always, we're constantly working to find ways to improve your experience with using our tools. See below for exciting updates on all these fronts.

As always, we love hearing from you, so please send your feedback and suggestions our way!

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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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Metrics from 1M sites

The number of performance metrics is large and increases every year. It's important to understand what the different metrics represent and pick metrics that are important for your site. Our Evaluating rendering metrics post was a popular (and fun) way to compare and choose rendering metrics. Recently I created this timeline of performance metric medians from the HTTP Archive for the world's top ~1.3 million sites:

Desktop metric timeline

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Lighthouse scores now available in your test results

In the year since Google rolled out Lighthouse, it's safe to say that "Will you be adding Lighthouse scoring?" is one the most common questions we've fielded here at SpeedCurve HQ. And since Google cranked up the pressure on sites to deliver better mobile performance (or suffer the SEO consequences) earlier this month, we've been getting that question even more often.

We take a rigorous approach to adding new metrics. We think the best solution is always to give you the right data, not just more data. So we're very happy to announce that after much analysis and consideration, we've added Lighthouse scores to SpeedCurve. Here's why – as well as how you can see your scores if you're already a SpeedCurve user.

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