I was analysing my page with Google PageSpeed, but was suprised to find that it was Google itself bringing errors:
The following cacheable resources have a short freshness lifetime. Specify an expiry of at least one week in the future for the following resources:
It seems Google's CDN files for +1 and Analytics have super-short expiry times, defeating the purpose of a CDN.
Why is this the case?
The purpose of a CDN is to distribute content, thereby putting it closer to your users and reducing the distance factor of the round trip time (RTT), and doesn't have an impact per say on the other two factors of the RTT, namely the speed of light and the time it takes the server to respond to the request.
In the general case, it is often static content, which can easily benefit from simple caching rules, that is put on a CDN since most CDNs don't have dynamic content generation capabilities, but there's nothing that says you can't put dynamic (ie. not easily cacheable) content on a CDN.
As far as why Google has elected to have relatively short cache expiry windows, there is always a tradeoff when it comes to any performance improvement, particularly with caching. The biggest trade-off for caching is the balance between the-most-recent-content-possible, and the resources it takes to generate the-most-recent-content.
This balance gets dramatically shifted by the scale of users that it impacts, and the resources you have available to spend on generating the content. In Google's case, I would guess that they have decided that a 30-minute window is long enough that they see a massive resource benefit, but is still short enough that they are happy with limiting themselves to a 30minute minimum response time for deploying new code.
Here is an interesting look at why caching things for even 5 seconds can be a worthwhile proposition, though you have to balance that with the update window, and I mentioned before.