LoadScriptsBeforeUI property of the ScriptManager class allows the user to control the nature of the download of the Ajax scripts. The boolean value indicate that the scripts are loaded before or after the page UI is loaded. If LoadScriptsBeforeUI is true( default value is true), all scripts will load before the page UI. If the value is false, the scripts will load after the page UI loaded. The end result is UI can quickly load and it will help the users to view the content first. So it is recommended to immediate display for your pages.
The syntax is
<asp:ScriptManager ID="ScriptManager1" runat="server"