The principal performance issues are on the database side (or maybe you could do some kind of caching or pre-loading), but your code could be made much clearer and shorter (there should be also some minor performance improvements):

string[] columnNames = new[] { "sub_id", "sibheadername" };
        
var categories = db.allcategories().ToList(); 
        
if (categories.Count == 0)
{
    return;
}
        
var dt = new DataTable();
dt.Columns.Add(columnNames[0]);
dt.Columns.Add(columnNames[1]);
        
foreach (var category in categories)
{
    var selectedCategories = db.selcategories(Convert.ToInt32(category.sub_id)).ToList();
    int count = Convert.ToInt32(selectedCategories.First().Column1);

    dr[0] = category.sub_id;
    dr[1] = count != 0
                ? category.sibheadername + " (" + count + ")" // btw. this is FASTER (in this case) than a StringBuilder!
                : category.sibheadername;
                        
    dt.Rows.Add(dr);
}
        
areaDropdown.DataSource = dt;
areaDropdown.DataTextField = columnNames[1];
areaDropdown.DataValueField = columnNames[0];
areaDropdown.DataBind();
areaDropdown.Items.Insert(0, "");

The principal performance issues are on the database side (or maybe you could do some kind of caching or pre-loading), but your code could be made much clearer and shorter (there should be also some minor performance improvements):

string[] columnNames = new[] { "sub_id", "sibheadername" };
        
var categories = db.allcategories().ToList(); 
        
if (categories.Count == 0)
{
    return;
}
        
var dt = new DataTable();
dt.Columns.Add(columnNames[0]);
dt.Columns.Add(columnNames[1]);
        
foreach (var category in categories)
{
    var selectedCategories = db.selcategories(Convert.ToInt32(category.sub_id)).ToList();
    int count = Convert.ToInt32(selectedCategories.First().Column1);

    DataRow dr = dt.NewRow();
    dr[0] = category.sub_id;
    dr[1] = count != 0
                ? category.sibheadername + " (" + count + ")" // btw. this is FASTER (in this case) than a StringBuilder!
                : category.sibheadername;
                        
    dt.Rows.Add(dr);
}
        
areaDropdown.DataSource = dt;
areaDropdown.DataTextField = columnNames[1];
areaDropdown.DataValueField = columnNames[0];
areaDropdown.DataBind();
areaDropdown.Items.Insert(0, "");