Web applications: Inaccurate "Visually complete" and "Speed index" metrics

maliborskaya — Thu, 25 Jan 2024 13:10:00 GMT

The "Visually complete" and "Speed index" metrics can be inaccurate due to the following reasons.

IFrames and Frames have their own JavaScript context, so it's not possible to access variables that are only available in another frame. DOM elements also must be available in a Frame/IFrame in which the RUM JavaScript is injected. These factors can lead to skewed metrics. Make sure you verify JavaScript variable or DOM element capture for these elements.
There may be a defined setTimeOut for XHR callbacks. Make sure you've enabled support for XHR (AJAX) frameworks, and that the specific XHRs haven't been excluded from monitoring.
Be aware that the Visually Complete module has no possibility to determine the render time of images that have been preloaded, since the listeners it would need to register are never called in such scenarios. This makes it impossible for the module to determine anything close to a render time of such images, so it either leverages mutation or resource load time. However, both of these metrics might be earlier than the actual render time of the image, especially if that image was preloaded and the page takes a long time to load.

article Web applications: Inaccurate "Visually complete" and "Speed index" metrics in Troubleshooting