Just copy and paste following codes to yourdomain.com/wp-content/themes/activated-theme/fucntions.php in the most below.
Example:
PHP
function performance( $visible = false ) {
$stat = sprintf( '%d queries in %.3f seconds, using %.2fMB memory',
get_num_queries(),
timer_stop( 0, 3 ),
memory_get_peak_usage() / 1024 / 1024
);
echo $visible ? $stat : "<!-- {$stat} -->" ;
}
add_action( 'wp_footer', 'performance', 20 );