Test Run Detail Report

In the test run detail, you can see the results of specific testing of one URL on your website.

The test run detail is designed for those technically inclined souls who crave the nitty-gritty of our speed testing using the Lighthouse tool.

What can you find in the test run detail?

1. The status of metrics for this test run.
2. Files with detailed logs.
3. Recommendations for speed optimization.

Test run detail.

Before diving deeper, ensure you understand the differences between synthetic, CrUX, and RUM data concerning web speed.

Familiarize yourself with how we test your websites in monitoring.

How to Access the Test Run Detail?#

You can display the test run detail in the following way:

1. Click on a point in the synthetic test results graph.
2. In the modal window that opens, click on the test time for the specific URL.
3. You will reach the standalone URL of the test run detail.

How to access the Lighthouse test run detail.

What Will You See in the Test Run Detail?#

Let's break down the individual parts of what you see on the screen and their usefulness:

1) Status of Metrics for This Test Run#

Lighthouse test result and the metrics within.

Here you will see:

1. Lighthouse score, i.e., the overall score calculated based on metrics.
2. The status of individual metrics. Besides the Core Web Vitals (LCP, CLS, TBT, but be careful, they are measured differently than in CrUX data), you also see the FCP metric and Speed Index.
3. The way the page is rendered during the Lighthouse test.

Bear in mind, the metric values here should not be your sole measure of your page's speed (that's what CrUX data is for). Treat synthetic data as a starting point for speed optimizations. They'll easily show whether your optimizations are working or not.

2) Downloadable Files#

At the top of the test run result, you can download JSON, Trace, and HAR files. What are they for?

🔒 Downloadable files are only available in PLUS tests.

Opening web load details in the Trace.cafe tool.

What do these mysterious acronyms JSON, Trace, and HAR mean, as seen in the image?

All three files are vital tools for any speed optimizer when needing to find causes of changes or opportunities for improvement.

JSON: Portable Output of the Lighthouse Report#

Open it, for example, in the Lighthouse Report Viewer or compare it with another test in Lighthouse Diff.

JSON will be handy for comparing two speed tests if changes occur. Or use them to store a test that shows unexpectedly positive or negative results.

Trace: Detailed Page Performance Report#

In our web speed consulting, we consider Trace the most crucial image of the actual real-world loading speed and subsequent page performance.

The Trace file can be opened in the Performance tab within Chrome DevTools or in the Trace.Café tool.

Michal Matuška discusses the Performance tab and other tools in Chrome DevTools in this video from Vzhůru dolů.

HAR: Page Download Report#

This is a supplementary report that shows potential issues in the page loading process in detail, including element prioritization, source loading, protocol used, or compression.

Open it, for example, in the Network tab inside Chrome DevTools.

3) Optimization Opportunities#

Besides the metric values, knowing what actions to take is crucial when you want to optimize websites.

Lighthouse has several recommendations for you.

Lighthouse recommendations should, of course, be taken with a pinch of salt. Some important recommendations might not be included here, and Lighthouse never considers the effectiveness of specific recommendations within your technological setup. Some of these recommendations may be unfeasible in a reasonable timeframe or might not have a significant impact.

Lighthouse Tool Error Messages#

In the test, you might see various error messages, which we will discuss here:

Timeout Exceeded#

The most common error is the incomplete measurement of the entire testing cycle due to exceeding the time limit that Lighthouse has when running on our infrastructure. This may manifest as errors like NO_TTI_CPU_IDLE_PERIOD or PROTOCOL_TIMEOUT.

In our experience, however, some of the five tests will always pass, which is why we test five times. If this does not happen, contact us.

Page Failed to Load#

Another possibility is errors where the page fails to load at all, such as:

• ERRORED_DOCUMENT_REQUEST – the requested page could not be reliably loaded.
• NO_FCP – the page rendered no content, and the FCP metric was not created.
• NOT_HTML – the page is not HTML, it was delivered with a MIME type of text/plain.

In such cases, ensure the page exists and, if so, that it is not blocking access to robots or hidden behind login or VPN.

Summary and Where to Go Next?#

The test run detail grants you truly detailed data from synthetic Lighthouse tests. It is highly beneficial for developers to learn how to work with this page.

On Vzhůru dolů, you will find a tutorial on how to optimize web speed using app.pagespeed.cz and specifically with the help of the test run detail.