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@Samrat Shrestha  This snipped work for you

<doctype html>
<html>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js">
</script>
    <head>
        <script type="text/javascript">
            $(document).ready(function() {
                 console.log("Time until DOMready: ",window.performance.timing.loadEventEnd-window.performance.timing.navigationStart);
             });
        </script>
        <!-- do all the stuff you need to do -->
    </head>
    <body>

    </body>
</html>


The ready event occurs after the HTML document has been loaded,
you not able to get page loading end time during window.onload (page loading).

https://stackoverflow.com/questions/3698200/window-onload-vs-document-ready
https://developer.mozilla.org/en-US/docs/Web/API/Navigation_timing_API#Examples
https://developer.mozilla.org/en-US/docs/Web/API/PerformanceTiming/loadEventEnd