Mysql行正在加速

I have VPS with next configuration:

CPU: 4 X 2.4 GHz

RAM: 8GB

160 GB SSD DISK

OS: CENTOS 7

CPANEL

400 MB/S PORT SPEED

Server is located in Lithunia, Vilnius.

I have created Tracking Pixel with PHP for my partner websites.

pixel.php

<?php

  $im=imagecreate(1,1);

  $white=imagecolorallocate($im,255,255,255);

  imagesetpixel($im,1,1,$white);

  header("content-type:image/jpg");

  imagejpeg($im);

  imagedestroy($im);


ob_start();
require_once 'config/DB.php';


// SET COOKIE 
if(!isset($_COOKIE['uid'])) {

    $guestcookie = sha1(time() . rand() . $_SERVER['SERVER_NAME'] . rand());
    setcookie('uid',$guestcookie);
    $_COOKIE['uid'] = $guestcookie;

}

if(isset($_COOKIE['uid'])) {

    $guestcookie = $_COOKIE['uid'];

}

    $ip = $_SERVER['REMOTE_ADDR'];

    $stmt = $db_con->prepare("INSERT INTO pixel1(uID, IP) VALUES(:uid, :ip)");

    $stmt->bindParam(":uid", $guestcookie);
    $stmt->bindParam(":ip", $ip);

    $stmt->execute();




?>

But when there is about 20,000 request to my tracking pixel in 1 minute, Pixel.php only inserting about 1,500 rows in MySql Table, and after that, my server and website works badly.