If you look at the portal site in firebug Net panel you will notice that the downloading of basic HTML takes less than 20-30 % time but major chunk of time is spent in downloading resources.
Basic problem is that the browsers use limited no. of connections for downloading resources from one host.Firefox 3.6, IE 8.0 use 6 and older browsers such as IE 6 or IE 7 use only 2 connections. So if your site has say 30 resources IE6 wont download all of them at the same time instead it will download 2 resources at a time. In one of the tests that i done i realized that the same site takes 1.5-2 times as long to render on IE 6 compared to IE 8 or Firefox
Both IE and Firefox allow you to change no. of parallel connections that should be used to download. This link has information on How do I configure Internet Explorer to download more than two files at one time?
You can make similar changes in Firefox by going to about:config and then changing value of
We cant expect users to change the registry settings or change firefox configuration for your site. But these are nice to know tricks when debugging performance issues