Is the content caching or not?

[pmeenan]

It looks very much like "browser caching" is enabled in W3 Total Cache because those are the default set of headers they use.

What you really want is for the browser to never even check to see if the resources have changed and always use the local copy immediately. When you have the version number on the resources then the version number changes only when the resource changes (and it becomes a new URL which the browser will fetch).

You want to avoid the browser checking for the resource at all because that takes just as long as downloading the resource in most cases.