fsockopen、fopen、file_get_contents なぜ fsockopen が最も速いのでしょうか?

fsockopen,fopen,file_get_contents fsockopen が最も速いのはなぜですか?
query_time_class.inc.php

<?php<br />
    class query_time_class{<br />
        <br />
        var $start_time;<br />
        var $end_time;<br />
        <br />
        function query_start(){<br />
            <br />
            $this->start_time = 0;<br />
            $this->end_time   = 0;<br />
            $this->start_time = $this->get_cur_sec();<br />
            <br />
        }<br />
        <br />
        function query_end(){<br />
            <br />
            $this->end_time = $this->get_cur_sec();<br />
            <br />
        }<br />
        <br />
        function get_query_time(){<br />
            <br />
            $time = floatval($this->end_time - $this->start_time);<br />
            $time = sprintf("%01.4f",$time);<br />
            return $time;<br />
            <br />
        }<br />
        <br />
        function get_cur_sec(){<br />
            <br />
            list($msec,$sec) = explode(" ", microtime());<br />
            $msec            = floatval($msec);<br />
            $sec             = floatval($sec+$msec);<br />
            return $sec;<br />
            <br />
        }<br />
        function out_put(){<br />
            <br />
            $time   = $this->get_query_time();<br />
            $output = "执行时间 {$time} 秒";<br />
            echo $output;<br />
            <br />
        }<br />
        <br />
    } <br />
<br />
<br />
?>

コードを実行