The CloudFront edge servers are very well connected to the internet and in general provide excellent speed around the globe. However, if it happens that you intranet goes through proxy servers or, as written earlier in this thread, if you are using a public DNS service (e.g. Google public DNS, OpenDNS), then this resolution could take more time and take you to a server that is going to be far away and slow to respond.
If this is the case it might be faster not to use CDN or use local CDN, as explained the custom CDN
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