I'm uploading a large file via ajax. The upload speed is fine but when the progress bar get's to 100%, there is a long wait and then i get this error

Failed to load resource: the server responded with a status of 504 (Gateway Time-out) - upload.php.

The file does appear in the destination folder but there isn't a server response.

This doesn't happen to smaller files. It seems to take long to move_uploaded_file from the Temp folder to the destination folder.

  var xhr, hUploadSpeed;
        function sendFile()
        {
            document.getElementById("serverresponse").innerHTML = "";//clear previous server response

            var url = "http://www.example.com/wp-content/themes/theme1/file-upload/upload.php";
            var formData = new FormData(document.getElementById("form1"));
            xhr = new XMLHttpRequest();

            xhr.upload.addEventListener('progress', uploadProgress, false);//EventListener for upload progress
            xhr.addEventListener('abort', uploadAbort, false);//EventListener for abort
            xhr.addEventListener('error', uploadError, false);//EventListener for error
            xhr.addEventListener('load', uploadThrough, false);//EventListener for completed upload

            xhr.open("POST", url, true);
            //xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded"); //no longer necessary here
            xhr.onreadystatechange = function()
            {
                if(xhr.readyState == 4 && xhr.status == 200)
                {
                    document.getElementById("serverresponse").innerHTML = xhr.responseText;
                }
            }

            xhr.send(formData); //Send to server
            hUploadSpeed = setInterval(UploadSpeed, 1000); //per seconds
        }

Upload.php

//You can access the uploaded files through $_FILES 
if(isset($_FILES["file1"]))
    $target_file = $_SERVER['DOCUMENT_ROOT']."/videos/".$_FILES["file1"]['name'];
    move_uploaded_file($_FILES["file1"]["tmp_name"], $target_file);

echo "File uploaded successfully.";

This works perfectly with smaller files 50mb but when i get to files around 300mb. It takes so long?

I have a dedicated server with this php.ini

upload_max_filesize = 1200M
post_max_size = 1200M
max_input_vars = 1000
memory_limit = -1
max_file_uploads = 20
max_execution_time = 7200
max_input_time = 7200
max_input_vars = 1000

Why does it take so long? How do i solve?