Perceived render time


The perceived render time represents how long the initial visible page takes to load.
With usual text / image size ratios, this largely depends on the load time of the slowest image within the page's visible range.


The perceived render time plugin is supported for all major browsers. To capture the perceived render time data, check the proper option in your System Profile's user experience section. See System Profile - User Experience for more information.

System Profile Preferences

Calculation of perceived render time

The perceived render time value is determined by the loading time of <img/> - elements on the page. When you enable the plugin, all image elements on the page are instrumented with a load-success, load-fail and load-abort handler. After the image loading process completes (successful or not), the image's load time is considered perceived render time. The load time becomes the new perceived render time if the following is true:

  • The image's load time is the slowest determined load time until now.
  • The image is within the initial browser window size.

Browser window size and images relevant for the calculation

As previously mentioned, only images within the initial browser window size can affect the calculated value.

The blue rectangle represents the initially visible part of the page. All image elements within this rectangle affect the perceived render time.

All images within a green rectangle are initially visible and can affect the perceived render time. Images within a red rectangle are outside the initial visible range and are irrelevant for the calculation.

Since there can always be more images affecting the shown screen, the final perceived render time can only be determined once the browser reaches the loadEventEnd.

User Action PurePath display

The perceived render time value displays as a thin line in the User Action PurePath. The left side of this line lists all elements that can affect the perceived render time calculation. All elements with terminated loading processes on the right side of the line do not affect the calculation.

To determine the exact value of the perceived render time and which image was used to calculate it, right-click on a PurePath's loading action to display the details. The browser window size displays as a reference.

Useable measures

The following Measures are available when using the perceived render time plugin.

Name Location Description
Browser window size UEM - Business Transaction Evaluation/Filter/Splitting values - User actions - Browser window size The browser window size (categorized)
Perceived render time UEM - User Actions - Perceived render time Measures the perceived render time in milliseconds.


Using the perceived render time plugin has these limitations.

  • Only image files directly embedded using the <img>-tag are calculation-relevant. Any images embedded by CSS, either by stylesheet rule or inline do not affect the calculation.
  • Entirely covered images (by elements with a higher z-index) are used for calculation, although they are not visible.
  • Invisible images (by CSS-rule visibility:hidden) also affect perceived render time.
  • If no images are detected within the visible screen area, no perceived render time value is calculated.