I'm checking a drupal site with YSlow and I get a "Grade E on Use a Content Delivery Network (CDN)" with this message:
"There are 43 static components that are not on CDN."
Just wondering if the cdn will help to improve this situation.
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It will only help if your files are actually on a CDN...if they are, then install that module, set it up, and you should see some performance benefits.
If not, then installing it will have zero effect, except to add a very slight extra overhead to your page build (when the module is included/hooks are run/etc).
If you can afford a CDN account/space (most of which as far as I know are paid services to be useful enough for a live site) and your site has adequate static content that will be used the module should help, just like @Clive says the files need to be physically present in your CDN to make the difference, else it will be just serving as a dead load whose hooks will be listened to on all page requests and events without any use. You could make the final call keeping in mind that what YSlow offers is only suggestions on improving performance and if your site has adequate hits to use a CDN that aids response.