ReportViewer 載入時間過長，加上Loading畫面

ReportViewer 載入時間過長，加上Loading畫面(使用ExtJS)

var myMask = new Ext.LoadMask(viewport, { msg: '處理中...' });
myMask.show();
var iframe = document.getElementById("mainContent");
iframe.onload = function () {
    myMask.hide();
  };
Ext.getDom('mainContent').src = "http://Site/Report.aspx"