Someone below mentioned doing requests for a large image that requires authentication. Short response time means the user isn't logged in (they got a 403), long response time means they downloaded the image and are logged in.

  const start = window.performance.now();
  const t = await fetch("https://example.com/asset_that_may_be_cached.jpg");
  const end = window.performance.now();
  if (end - start < 10/*ms*/) {
    console.log("cached");
  } else {
    console.log("not cached");
  }

> In that case, the browser would always load the asset (it is not cached).

Agreed, if the cache is partitioned per domain AND the current domain has not requested the resource on a prior load. If the cache is global, then the asset will be loaded from cache if it is present: https://developer.mozilla.org/en-US/docs/Web/API/Request/cac...

> So the rule would be that only stuff that is directly in the <head> may be cached (or stuff that is on the same domain).

You could be more precise here: with a domain-partitioned cache, all resources regardless of domain loaded by any previous request on the same domain could be cached. So if I load HN twice and HN uses https://example.com/image.jpg on both pages, then the second request will use the cached asset.

Ah right, the thread is becoming long :)

> So if I load HN twice and HN uses https://example.com/image.jpg on both pages, then the second request will use the cached asset.

Good point!