Web applications sometimes appear to lack smoothness when the the application isn't keeping up with the refresh yet. This is also referred to as jankiness.  

According to jankfree.org:

Jank is any stuttering, juddering or just plain halting that users see when a site or app isn't keeping up with the refresh rate. Jank is the result of frames taking too long for a browser to make, and it negatively impacts your users and how they experience your site or app.

To see an example, go to https://googlechrome.github.io/devtools-samples/jank/

Enable your script to measure Jankiness

  1. Add the necessary capabilities to execute a performance script, as described in Measure Page Load Performance using Test Automation.
  2. Unlike the page load performance, it is up to the script to decide when to measure jankiness. Add the following to your script at the right places you want to measure jankiness:

For details, see Test Command.

Reporting Jankiness

When your test completes, it generates a set of metrics and that you can access through the Performance tab of the Test Details page. If you detect a regression in your website's performance, you can download a Full Trace report, or you can use the Chrome DevTool, which records Javascript method signatures in a hierarchical view for each thread in each process, to get a better idea of how the Chrome browser is interacting with your website.

The full Performance Report provides you access to all jank metrics as well as area charts showing the Frames Per Second (FPS) CPU and Heap memory used during the jank test