According how this flag is defined and used (mainly for XHR to not be blocked on head css/js loads) it is not a perf win to consider them as a cache priority loads. Cache is not blocked by any priority loading, so only affect is that the XHR will load a bit later, while expected to not have any major impact on page load and user experience performance.
Created attachment 8365259 [details] [diff] [review] v1
Michal, this should land before we do the temporary m-c testing. Can give the one line-review a priority please?