By default, when you install new plugins in a WordPress blog that plugin may add custom CSS styles or Javascript functions that could make your page a little loader to load to your visitors. The same happens with many popular WordPress themes.
You can easily see how long it takes to load by using for example Firebug with the Network tab enabled.
Recently I found this interested article from Justin Tadlock that realized me a quick and elegant way to disable scripts or styles enqueued by WordPress plugins or themes.
His solution allows someone to unqueue styles and css by adding a couple of code lines at functions.php
A few examples of plugins that add custom css styles could be WP-PageNavi, CForms, WP-DownloadManager, etc.
