Filter recors and fill in drop down with huge sql data

Hi I have a data of more that 500000 records i want to search data and fill in other dropdown list i am using ListSearchExtender with dropdown its working buts it is very slow
when i search and select first dropdown it should fill another dropdown and when i search and select second dropdown it should fill frst dropdown..
<b>is there any alternative way to filter data and fill ??????????????</b>

here is my code
<td class="style43" colspan="3"> <asp:DropDownList ID="ddlICD10" runat="server" AutoPostBack="True" onselectedindexchanged="DropDownList1_SelectedIndexChanged" > </asp:DropDownList> <asp:ListSearchExtender ID="ddlICD10_ListSearchExtender" runat="server" Enabled="True" TargetControlID="ddlICD10"> & ...