Detailed information posted on the forum: http://www.zabbix.com/forum/showthread.php?t=26037

In short:

I have noticed that browsers repeatedly revalidate static objects (images, css files, javascript files) on the Zabbix status/graph pages. Meaning that they make a lot of unnecessary GET requests again and again, for objects that haven't changed and should actually come from the browser's cache. This results in significant overhead and slow loading pages.

This behaviour is triggered by the use of the Javascript location.reload() call for automatically refreshing the pages. This forces the browser not only to reload the page itself, but also revalidate every object on the page. This can be correct however by using a different page reload mechanism:

Replace the line (#41):

location.reload();

by:

window.location.href = window.location.href;

Speed ups are pretty big. For more detailed information please see the forum post (if needed I can copy-paste the post's content here).