After playing around with Firebug a little I found out that in our project the most costly thing in the page loading process is loading the dojo .js-files. For some reason they are included in the xhtml theme too. If I substitute
<s:head theme="xhtml" /> with <link rel="stylesheet" href="/ikaros-webgui/struts/xhtml/styles.css" type="text/css"/> leaving out the dojo stuff, pages are loading really fast again. Is there any reason besides the client side validation that the dojo includes are needed for the xhtml theme?