Why do the Matomo Page Performance report show some days with 0s Processing Time and others with 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 (Eg. 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 which 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.
   Unfortunately, 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.