Downloadscript extremely slowly

Question

I've written a download script with javascript and php. It works, but if i want to download a large file(example 1GB zip file) it has tooo long to end up with the request. I think it has something to do, that i read the file. If it's this, any idea how to get it faster?
Notice : I need a header, cause of force downloading like images, pdfs, any kind of filetype.

JS is very simple. Look at this:

function downloadFile(file){
    document.location.href = "script.php?a=downloadFile&b=."+ file;
}

PHP is simple yet:

function downloadFile($sFile){
    #Main function
    header('Content-Type: '.mime_content_type($sFile)); 
    header('Content-Description: File Transfer');
    header('Content-Length: ' . filesize($sFile)); 
    header('Content-Disposition: attachment; filename="' . basename($sFile) . '"');
    readfile($sFile);
}

switch($_GET['a']){

    case 'downloadFile':
        echo downloadFile($_GET['b']);
        break;
}

Answer 1

I guess buffering is the issue for the large files. Try reading your file in small chunks (like a megabyte) and call flush function to flush the output buffer after printing each chunk.

EDIT: eh, okay, here's the code example you should try:

function downloadFile($sFile){
    #Main function

    if ( $handle = fopen( $sFile, "rb" ) ) {
        header('Content-Type: '.mime_content_type($sFile)); 
        header('Content-Description: File Transfer');
        header('Content-Length: ' . filesize($sFile)); 
        header('Content-Disposition: attachment; filename="' . basename($sFile) . '"');

        while ( !feof($handle) ) {
            print fread($handle, 1048576);
            flush();
        }
        fclose($handle);
    } else {
        header('Status: 404');
        header('Content-Type: text/plain');
        print "Can't find the requested file";
    }
}