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requestStart fires after domainLookupEnd, secureConnectionStart and connectEnd as defined by the Processing Model of the PerformanceResourceTiming interface (URL: https://www.w3.org/TR/resource-timing-1/#dom-performanceresourcetiming-requeststart ). As this page defines TTFB time as including "DNS lookup and establishing the connection using a TCP handshake and SSL handshake if the request is made over https." I suggest "TTFB= responseStart" or "TTFB= responseStart - navigationStart" instead of "TTFB= responseStart - requestStart". By the way, Google calculate the TTFB in their Chrome Extension "web vitals" as "getTTFB=responseStart" ****extract form URL: https://github.com/GoogleChrome/web-vitals/blob/master/src/getTTFB.ts*** export const getTTFB = (onReport: ReportHandler) => { const metric = initMetric('TTFB'); afterLoad(() => { try { // Use the NavigationTiming L2 entry if available. const navigationEntry = performance.getEntriesByType('navigation')[0] || getNavigationEntryFromPerformanceTiming(); metric.value = metric.delta = (navigationEntry as PerformanceNavigationTiming).responseStart; metric.entries = [navigationEntry]; onReport(metric); } catch (error) { // Do nothing. } }); }; *****
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I suggest "TTFB= responseStart - navigationStart" instead of "TTFB= responseStart - requestStart".
requestStart fires after domainLookupEnd, secureConnectionStart and connectEnd as defined by the Processing Model of the PerformanceResourceTiming interface (URL: https://www.w3.org/TR/resource-timing-1/#dom-performanceresourcetiming-requeststart ). As this page defines TTFB time as including "DNS lookup and establishing the connection using a TCP handshake and SSL handshake if the request is made over https."
By the way, Google calculates the TTFB in their Chrome Extension "web vitals" as "getTTFB=responseStart"
*extract form URL: https://github.com/GoogleChrome/web-vitals/blob/master/src/getTTFB.ts
export const getTTFB = (onReport: ReportHandler) => {
const metric = initMetric('TTFB');
afterLoad(() => {
try {
// Use the NavigationTiming L2 entry if available.
const navigationEntry = performance.getEntriesByType('navigation')[0] ||
getNavigationEntryFromPerformanceTiming();
metric.value = metric.delta =
(navigationEntry as PerformanceNavigationTiming).responseStart;
metric.entries = [navigationEntry];
onReport(metric);
} catch (error) {
// Do nothing.
}
});
};