Edge browser freezes - drop-down list

Question

I'm working on an legacy ASP.NET application which runs on IE Compatibility mode. In one of the page it loads 15000 records to a drop-down list.

sample code:

foreach (DataRow dr in ds.Tables[0].Rows)

                {

                    ddlFind.Items.Add(new ListItem("DD " + dr["EMPID"].ToString() + " - " + dr["EMPNAME"].ToString(), dr["EMPID"].ToString()));

                }

For most of the users it works fine, but for few users the page freezes (blue-spinning wheel) once the user clicks on drop-down list. Also for those users it works fine in InPrivate window. We took actions like browser uninstall/reinstall, repair, updated to latest version, cleared cache, cookies, history files etc but in vain. Need your help.