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Isn't using already cached assets one of the reasons to use a CDN for popular scripts?

Except that the chance of the asset you are requesting being cached is pretty minimal. See https://github.com/h5bp/html5-boilerplate/pull/1327 for links and discussion about this. The conclusion seemed to be that is it likely that a CDN does provide some benefit, but more from a localization of content and offloading the bandwidth, rather than a speed or cache hit benefit.

I might be missing something, but that link really only talks about the potential benefit of a file already being cached locally on the end-user's device.

There's still a significant benefit in just storing the file closer to the end-user, rather than in a single central location, as the CDN is likely to have both lower latency and have higher bandwidth than the source website.

Now, if your file was so unique and rarely accessed that it wouldn't even be cached on the CDN, then I'd agree with those findings.

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