C# - Speed Up Loading Data From Database?

I'm using Webservice which will return the data from the database through datatable and I'll convert the datatable into byte array. In front end, I'll reconvert the bytearray to datatable and used ajaxloader to display it in the form.. It is dynamic loading. So, for each click, it is taking 10 seconds to retreive the data regardless of the size of the data. So, I used static datatable and I've loaded all data in that datatable in page load event. But, no reaction. It is taking only same time. Even, there is no data to retreive, the ajax loader is going on loading for 10 seconds. Problem is with Ajax or my webservice??

My Code for listboxclick event

protected void listboxcity_SelectedIndexChanged(object sender, EventArgs e)
{
string sqlvalue = string.Empty;
//Thread.Sleep(200);
for (int i = 0; i < listboxcity.Items.Count; i++)

[Code]....