Why is it slow slow navigating away from our Facebook app when we implement Application.ExternalEval

We put in this example code in our game which works but it takes about ten seconds to navigate away from our page now. It is the worst in Internet Explorer. Any idea why?

 string injection =
             "var headerElement = document.createElement('div');" +
                 "headerElement.textContent = ('Check out our other great games: ...');" +
                 "var body = document.getElementsByTagName('body')[0];" +
                 "var insertionPoint = body.children[0]; " +
                 "body.insertBefore(headerElement, insertionPoint);";
         Application.ExternalEval(injection);