Google just released a Firefox/Firebug add-on called Pagespeed. It is basically a tool that will help you improve the loading performance of your website. After you run it, you will see a list with all the factors that would make your site load faster, with an image illustrating whether or not your website checks for each.
There are some similar plugins available for Firefox, but this one seems to be more robust. It checks for a wide range of optimization steps, including proxy caching, DNS lookups, CSS combination, Javascript loading and parallel downloads. Worth a try.
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