Last-Modified header is changed, even if the resource hasn't changed Unless I'm mistaken, Drupal is changing the "Last-Modified" HTTP header for a page any time the page cache is regenerated - even if the raw content remains identical.  This means that clients end up having to re-download exactly the same information any time the page cache is regenerated even for pages that have not changed at all.

I realize that I can improve this situation by trying to engineer things such that the page caches are not regenerated unless the resource has changed, but is there any way I can set things up such that the Last-Modified header genuinely reflects whether or not the content has changed?  This would seem to me to be a more robust solution.
The code that is causing the issues you're describing lives in drupal_serve_page_from_cache(). Instead of using a hash of the contents of the cache; Drupal uses the $cache-&gt;created value. Looking over everything hacking core is the only option at the moment. I'll see if there can be a way of not hacking core if one has total control of the cache backend; if there is a way look for an update inside of https://www.drupal.org/project/apdqc