Why do the Matomo Page Performance reports show 0s processing time or very high processing time?

With Matomo 4, Page Performance tracking is included by default in the Matomo JavaScript tracking. In some cases these reports will look quite different when comparing day to day reports with each other.

When viewing the Page Performance reports in Matomo you may even come across some days that show for example 0s Avg. DOM Processing Time for some days while other days show the same metric as very high (e.g, more than 8s for the same Avg. DOM Processing Time.

This can be caused by several different factors:

1. Devices that have cached content loading the page quickly
Many browsers are able to very quickly or nearly instantaneously load content that has already been downloaded and processed. This is done automatically by many different browsers when it receives a 304 Not Modified response to a request.

2. New content that is overriding cache-control attributes forcing a re-download and reprocessing of page content on some days
When your website was recently updated or new content was added, it may be expected that the processing time for this new content will be higher because the browsers may need to download updated content or scripts.

3. Older devices that load pages slower
Older devices that load the page a lot slower than others, causing some days to have a much higher Avg. DOM Processing time. This is especially true for pages or websites that receive very little traffic. It is important to remember that the Avg. Performance metrics measured by Matomo can be heavily impacted by slow devices for low traffic pages and websites.

4. Devices that do not support DOM processing Time or Network Time tracking
Not all devices support all of the tracking methods used for the Page Performance reports. When there is no data present for a specific performance metric, Matomo will show this metric as 0 in the reports.

5. Extremely high or 0 second page timing values
Some devices and browsers report inaccurate or unusually high DOM processing time values that do not reflect actual page load behaviour. Some browsers could report timing values of 0 seconds when performance timing data is unavailable or blocked, or report a processing time of several hours instead of a few hundred milliseconds. Read more on common reasons for slow pages.

Because Matomo reports average values, even one extreme outlier can significantly increase the reported average on low-traffic pages or websites. For example:

• 10,000 visits with a DOM processing time of 250 ms results in an average of 0.25 seconds.
• 10,000 visits with a DOM processing time of 250 ms plus one incorrect value of 4 hours results in an average of 1.69 seconds.

These unusually high values are typically caused by browser, device, tab suspension, background processing, or timing anomalies rather than actual page performance issues.