为什么ajax会给出迟到的结果？

Question

I was trying to get value from ajax and following jquery using in script for ajax:

$("#disc-num").keyup(function(){
    var discountCode = $("#disc-num").val();
    $("#jps-coupon-code").val(discountCode);
    var ticketId = <?php echo $_GET['ticketId'] ?>;
    $.ajax({
        type: 'GET',
        url: 'http://example.com/includes/registration-ajax.php',
        data: {myDiscCode:discountCode, myId:ticketId},
        success: function(more){
            $("#myDiscount").val(more);
        }
    });
});

and my ajax file(registration-ajax.php) is below:

if(isset($_POST['myDiscCode'])){
 $discCode = $_POST['myDiscCode'];  
 $tickId = $_POST['myId'];
 $discQuery = "SELECT * FROM jps_coupon WHERE coupon_code='$discCode' AND ticket_id='$tickId'";
 $Query = mysqli_query($con, $discQuery);
 while($my=mysqli_fetch_array($Query)){
     $cDiscount = $my['coupon_discount'];
 }
 echo $cDiscount;
}

and my HTML file is below:

<div class="input-group add-on" id="discount-div">
    <input type="text" class="form-control" id="disc-num" style="border-top-right-radius:0 !important;border-bottom-right-radius:0 !important;">
    <div class="input-group-btn">
    <button class="btn btn-default" type="button" id="apply">Apply</button>
</div>
</div>

Above code sometime works and sometime not, problem is ajax is taking time to give result in success. so please check my ajax and tell me why success:function responding late.