This is weird for sure. I can't see why the browser is issuing the If-Modified-Since requests for some resources and then getting the other resources from the cache. I can see the resources in the browser cache (chrome://cache/), yet the browser is still issuing the request and getting the 304 response. I am 100% sure everything was coming from the cache when we made the changes to set the Cache-Control header, so not sure what has changed. We want to eliminate those requests.