PerformanceResourceTiming deliveryType explainer
Resources on the web are usually fetched via HTTP (from the origin server), but in some cases they are known to be delivered from a cache or other buffer, in a way that affects loading performance. Insight into this is useful for authors understanding when these caches accelerate resource loading.
deliveryType attribute on
PerformanceResourceTiming addresses this, with the following values:
- No more specific delivery type applies, most likely because the resource was fetched from its origin server. Also returned if the user agent doesn't wish to disclose the use of a cache, for security or privacy reasons.
- The resource was served from the cache (i.e., the user agent's HTTP cache).
- The resource was prefetched for navigation on another page (e.g., using speculation rules) and served from the prefetch buffer when navigation subsequently occurred.
w3c/resource-timing#303 discusses how consuming a preload ought to be exposed in the future; if it is, using another
"preload") would be viable. This is not yet proposed as of June 2023, however.