Resource Timing

Resource Timing is a browser API that, for every file a page loads — an image, a script, a font — exposes the precise timing of each stage of its retrieval.

The value of this API is that it breaks a page's slowness down to the level of individual resources: you can see which image arrived late, which third-party script pushed render back, and how much each resource spent in DNS, connection and download. But it has two limits. First, volume: a page may have dozens or hundreds of resources, so sending the timing of all of them for every visit is expensive, and tools usually either sample or send only a count and summary. Second, resource privacy: for files coming from another domain, the browser hides the timing detail unless an explicit permission header is present, to prevent information leaking. So a full resource-by-resource analysis often needs coordination with the resource's provider.