A "Loading..." label is displayed in the upper-left corner whenever a CallBack takes more than 100 ms (ASP.NET WebForms, ASP.NET MVC, Java).
JavaScript
In the JavaScript version, you can display the label using loadingStart() method (e.g. when invoking an HTTP request) and hide it using loadingStop() (when the HTTP response is received).
Styling
The styles are defined using *_loading CSS class in the theme. See also CSS Classes.
ASP.NET WebForms
The loading label is enabled by default (LoadingLabelVisible is set to true).
Custom HTML
You can override the Loading label HTML using LoadingLabelText property.