Just like other validators, there is a webpage that allows you to plug-in your page's URL and let it do some crunching. Google also released PageSpeed as an extension for it's Chrome's Developer Tools and extends the FireBug extension for FireFox.
Your results are broken down into three categories: High, Medium, and Low priority. Each suggestion will provide a description of what can be optimized and how. For example, if you do not have any kind of compression running, it'll analyze how much data transfer you could save with it, the same with minifying your CSS, etc.
Want something more? Google also provides an Apache module, called mod_pagespeed. A lot of those small tweaks can be handled automatically for you by the module, saving you time and effort.
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