DNS timing
Browsers do not let a page time an arbitrary DNS resolver. This check reads the Resource Timing lookup interval for one same-origin request, and only when that interval is actually present.
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How this measurement works
domainLookupEnd minus domainLookupStart is shown when both timestamps are positive and the difference is greater than zero. A cached name, a reused connection, or a cross-origin restriction produces an unavailable state. Total HTTP time is never labeled as DNS.