优化phpmailer的邮件发送过程

I am trying to send mails using phpmailer and code is working fine. However when I see the CPU usage history in task manager I could see it is fluctuating rapidly between 0% to 100%. And RAM usage is constant around 4.65GB of 6GB.

I was able to send 500 emails in 15 minutes or so and I feel its bit slow because my CPU usage is fluctuating a lot, may be I am wrong in my assumption.

1. Can anyone help me in optimizing max usage of my system and fasten the process?

2. Also I have created a column in table email_sent to check how many mails have been sent and avoid any duplication but the code is not updating the database and giving up! (so it is commented out below).

   <?php
   //this code sends mails in HTML format using emailids from database. It also sends attachment if needed.
   error_reporting(E_ALL);
   //error_reporting(E_STRICT);
   ini_set("display_errors", "on");
   ini_set('max_execution_time', 30000000); //300 seconds = 5 minutes
   ini_set('memory_limit', '6500M'); //Maximum amount of memory a script may consume (128MB by default)
   date_default_timezone_set('UTC');
   
   require_once('../class.phpmailer.php');
   //include("class.smtp.php"); // optional, gets called from within class.phpmailer.php if not already loaded
   
   $mail                = new PHPMailer();
   $body                = file_get_contents("contents.html");
   //$body                = preg_replace('/[\]/','',$body);
   $body                = preg_replace('/IDontKnowWTFThisFunctionIsdoing/','',$body);
   
   $mail->IsSMTP();        // telling the class to use SMTP
   $mail->Host          = "ssl://smtp.gmail.com";
   $mail->SMTPAuth      = true;                  // enable SMTP authentication
   $mail->SMTPKeepAlive = true;                  // SMTP connection will not close after each email sent
   $mail->Host          = "ssl://smtp.gmail.com"; // sets the SMTP server
   $mail->Port          = 465;                    // set the SMTP port for the GMAIL server
   $mail->Username      = "myemail@gmail.com"; // SMTP account username
   $mail->Password      = "password";        // SMTP account password
   //$mail->SetFrom    ('myemail@gmail.com', 'me');
   $mail-> From         = "myemail@gmail.com";
   $mail->FromName      = "me";    
   $mail->AddReplyTo   ('myemail@gmail.com', 'me');
   
   $mail->Subject       = "Subject";
   
   @MYSQL_CONNECT("localhost","root","");
   @mysql_select_db("database"); 
   $startnum1 = 501;
   $endnum1   = 5000;
   //$query     = "SELECT emailid FROM test WHERE email_sent = 0 Limit $startnum1,$endnum1";
   $query     = "SELECT emailid FROM test Limit $startnum1,$endnum1";
   $result = @MYSQL_QUERY($query);
   echo "message sending in process";
   while ($row = mysql_fetch_array ($result)) {
     $mail->body       = $body;
     $mail->AltBody    = "To view the message, please use an HTML compatible email viewer!"; // optional, comment out and test
     $mail->IsHTML(true);
     $mail->MsgHTML($body);
     $mail->AddAddress($row["emailid"], $row["emailid"]);
     //$mail->AddStringAttachment($row["emailid"], "contents.html");
   
     if(!$mail->Send()) {
       echo "Mailer Error (" . str_replace("@", "&#64;", $row["emailid"]) . ') ' . $mail->ErrorInfo . '<br />';
     } else {
       //$query1 = "UPDATE test SET Email_Sent= 1 WHERE EmailID= emailid Limit $startnum1,$endnum1";
       //$result1 = @MYSQL_QUERY($query1);
       echo "Message sent to :" . $row["emailid"] . ' (' . str_replace("@", "&#64;", $row["emailid"]) . ')<br />';
     }
     // Clear all addresses and attachments for next loop
     $mail->ClearAddresses();
     //$mail->ClearAttachments();
   }
   [CPU history] (http://s3.postimg.org/v99ucpq6p/Capture.jpg)
   [System information] (http://s3.postimg.org/3n72s16tt/Capture1.jpg)
   

   ?>