DropDown Opens with a Delay

In case of RadComboBox with a high number of Items a brief delay is observable when the drop down is opened for the first time in a page life cycle.The delay is caused by the so called ‘lazy initialization’ mechanism - Item objects are initialized in the latest possible moment in the control life cycle. This is the first time Items are accessed with the get_items() method, which happens automatically right before the drop down opens.

You could avoid this delay, or more specifically 'move' it to the initial page load, by calling get_items early in the page life cycle either for all RadComboBoxes on the page or for individual ones.

The first approach uses the pageLoad function as shown below:

function pageLoad(sender, eventArgs) {
   //get an array of all RadComboBoxes on the page
   var comboBoxesOnPage = Telerik.Web.UI.RadComboBox.ComboBoxes;

   for (var ix = 0; ix < comboBoxesOnPage.length; ix++) {
       comboBoxesOnPage[ix].get_items(); //force Item initialization
   }
}

The second approach uses the OnClientLoad event of a single RadComboBox:

<script type="text/javascript">

   function onLoad(sender, eventArgs) {
       sender.get_items(); //force Item initialization
   }
</script>

<telerik:RadComboBox RenderMode="Lightweight" ID="RadComboBox1" runat="server" OnClientLoad="onLoad" />