Downside of using CNAME in CDN?

[pmeenan]

Slight but as long as you have a long TTL on the CNAME it will get cached by the various ISP's DNS resolvers.

Having a CNAME basically adds another query that the DNS servers need to do - they need to query your DNS resolver first and then go query the CDN provider's resolver.

We're generally talking on the order of 10's of ms but it could be slower depending on what you are using for your site's DNS and how well distributed it is.