Use a variable created in get_header to calculate stuff in wp_footer

You need to redeclare the variable in your end_time() and also you don’t need the user to be logged in.. You can try the below answer.

// LOAD TIME CHECKER
function start_timer() {

    global $time_start;
    $time_start = microtime(true);
}
add_action('wp_head', 'start_timer', 1);

function end_time() {
      //try redeclaring the var 
       global $time_start;
    echo '
        <div class="clearfix"></div>
        <div id="queryTime" style="position: fixed; bottom: 30px; left: 30px;">
            <ul class="li"><span class="label label-success fs13">Start Time: '.$time_start.'</span></ul>
            <ul class="li"><span class="label label-success fs13">Query Took: '.number_format( microtime(true) - $time_start, 10).'</span></ul>
        </div>
    ';
}
add_action('wp_footer', 'end_time', 100);