Deprecated: This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes. Avoid using it, and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.
Warning: This interface of this property is deprecated in the Navigation Timing Level 2 specification. Please use the PerformanceNavigationTiming interface instead.
The legacy PerformanceTiming.responseStart read-only property returns an unsigned long long representing the moment in time (in milliseconds since the UNIX epoch) when the browser received the first byte of the response from the server, cache, or local resource.