Rule 2 says to use a content delivery network (CDN). The score for this rule is computed by checking the hostname of each component against the list of known CDNs. Unfortunately, the list of “known CDNs” are the ones used by Yahoo! Most likely these are not relevant to your website, except for potentially yui.yahooapis.com. If you want an accurate score for your website, you can add your CDN hostnames to YSlow using Firefox’s preferences. Here are the steps to follow:
Go to “about:config” in Firefox. You’ll see the current list of preferences.
Right-click in the window and choose New and String to create a new string preference.
Enter extensions.yslow.cdnHostnames for the preference name.
For the string value, enter the hostname of your CDN, for example, mycdn.com. Do not use quotes. If you have multiple CDN hostnames, separate them with commas.
If you specify CDN hostnames in your preferences, they’ll be shown under the details for Rule 2 in the Performance view.