functions.php

<?php

function response_time() {
    echo "<script>
        window.addEventListener(
            'load',
            function() {
                setTimeout(
                    function() {
                        if ( window.performance && window.performance.timing && window.console ) {
                            var timing = window.performance.timing;

                            console.log(
                                'Response time: %d ms',
                                parseInt(timing.responseEnd - timing.fetchStart)
                            );
                        }
                    },
                    0
                );
            },
            false
        );
    </script>";
}

add_action(
    'wp_footer',
    'response_time'
);