Imagine the following approach:
Combobox items are added in the Page_Load event. The items are added from a huge datasource. The code is placed in if (!Page.IsPostBack) block RadComboBox is in a callback mode - the items are also added in the ItemsRequested event of the combobox.
The code in the if (!Page.IsPostBack) block is executed upon each postback. This is what makes the combobox performance slow.
Telerik RadComboBox performs a "light" callback to the server - no ViewState information is sent to the server and the Page.IsPostBack property is set to False when the combobox is in a callback mode.
In addition, the Page_Load method is called before the ItemsRequested event. Therefore, all page initialization code is fired, too.To avoid re-executing of the code in the Page_Load event, you should check for both Page.IsPostBack and Page.IsCallBack:
protected void Page_Load(object sender, EventArgs e)
if (!Page.IsPostBack && !Page.IsCallBack)