Unfortunately there are issues running QVS 11 AJAX on IE8.
IE8 was developed by Microsoft prior to the advent of HTML5.
I believe that QlikView 11 and up uses HTML5 and thus the poor performance with IE8.
If you cannot get the browser upgraded then try to get the Client to use another browser like Chrome or Firefox. (11.2 works well with Chrome).
Using another browser or upgrading IE will produce a big improvement in response.