How to Accurately Measure Load Times with the Performance.now() API

Performance API can measure the time between two predefined markers by defining a start and an end point.

Example: var start = performance.now(); The difference from using Date timestamps is twofold: performance.now() provides millisecond values with sub‑millisecond (three‑decimal) precision, whereas Date.now() returns integer milliseconds; and performance.now() measures time elapsed since the page navigation start, while Date gives the current epoch time.

Example: measuring an image's load time.

var start = performance.now();
document.getElementsByTagName("img")[0].addEventListener("load", function() {
    var end = performance.now();
    console.log((end - start) + "ms");
}, false);