With the Resource Timing V2 coming to Firefox (it's in FF45) it gives us a really cool thing: By checking the transferSize of a resource we can know if it is picked up from cache or fetched from the server (transfer size is 0 when it's from cache). Chrome has it on its 2do list hopefully for Q2.
We need to think of how we can use it. Should we collect how many of the requests that are cached? Should we categorize Navigation Timing data as cache hits or not?
I'll add upstream as we probably want Chrome on board before we start using it, but we can prepare and think of how we best can use it before it's implemented.