This class primarily can be used in conjunction with StackPanel, DisclosurePanel, and TabPanel when the child panels contain relatively heavy weight contents. It can a speed your GWT UI load time if LazyPanel is used wisely.
Check out following links for more information:
API documentation link: http://google-web-toolkit.googlecode.com/svn/javadoc/1.6/com/google/gwt/user/client/ui/LazyPanel.html
Example link: http://ukitech.blogspot.in/2009/07/example-for-gwt-lazy-panel.html