Make wp_generate_attachment_metadata fast (http://www.wpquestions.com/question/showChrono/id/9671)

heavy.php

<?php

/**
 * heavy.php
 * 
 * Script for generating meta data from POST inputs ( url, post_id, key )
 * 
 * version 0.0.1
 */

// Include WordPress:
define( 'WP_USE_THEMES', false );
require( './wp-blog-header.php' );   // Edit this path to your needs, (relative or absolute path)

// Secret key:
$my_secret = 'ek4le8c4wh'; // Edit this to your needs.

// POST input parameters:
$input_post_id = filter_input( INPUT_POST, 'post_id', FILTER_SANITIZE_NUMBER_INT );
$input_url     = filter_input( INPUT_POST, 'url', FILTER_SANITIZE_URL );
$input_secret  = filter_input( INPUT_POST, 'secret', FILTER_SANITIZE_NUMBER_INT );

// Validate input:
if( $mysecret != $input_secret 
    || empty ( $input_url ) 
    || empty ( $input_post_id ) 
)
{
    die('No Access');
}

//----------------
// Your script:
//----------------

// Get the path to the upload directory.
$wp_upload_dir = wp_upload_dir();

// Image URL
$url = $input_url;

// Full PATH to image in Upload Directory 
$filename = basename( $url );
$file = $wp_upload_dir['path'] . "/$filename";

// Full PATH to image in Upload Directory 
$url2 = $wp_upload_dir['url'] . "/$filename";

// The ID of the post this attachment is for.
$post_id = $input_post_id;

// Check the type of tile. We'll use this as the 'post_mime_type'.
$filetype = wp_check_filetype( $file );

// Prepare an array of post data for the attachment.
$attachment = array(
'guid'	=> $url2,
'post_mime_type'	=> $filetype['type'],
'post_title'	=> preg_replace( '/\.[^.]+$/', '', $filename ),
'post_content'	=> '',
'post_parent'	=> $post_id,
);

// Make sure that this file is included, as wp_generate_attachment_metadata() depends on it.
require_once( ABSPATH . 'wp-admin/includes/image.php' );

// Insert the attachment.
$attach_id = wp_insert_attachment( $attachment, $file, $post_id );

if ( !is_wp_error($attach_id) ) {
    // Generate the metadata for the attachment, and update the database record.
    $attach_data = wp_generate_attachment_metadata( $attach_id, $file );
    wp_update_attachment_metadata( $attach_id, $attach_data );
}	

The Gravity Form part

<?php

//Multi upload field - Insert images into WP media library and link them to post ID

add_action( 'gform_after_submission_20', 'my_add_post_attachements', 10, 2 ); 

function my_add_post_attachements( $entry, $form )
{
    $submit = array();
    $submit = $entry["38"]; //The field ID of the multi upload
    $submit = stripslashes($submit);
    $submit = json_decode($submit, true);

    // Edit this to your needs:
    $my_secret           = 'ek4le8c4wh';
    $my_heavy_script_url = 'http://example.com/heavy.php';
    
    // Loop:
    if( function_exists( 'demo_async_curl_post' ) )
    {
        foreach ( $submit as $key => $value )
        {
            $post_fields = array( 
                'url'     => $value, 
                'post_id' => $entry['post_id'], 
                'key'     => $my_secret 
            );
            
            demo_async_curl_post( $my_heavy_script_url, $post_fields ); 
        }
    }

}


if( ! function_exists( 'demo_async_curl_post' ) )
{
    function demo_async_curl_post( $url = '',$postfields = array() )
    {
        $curl = curl_init();
        curl_setopt( $curl, CURLOPT_URL, $url);
        curl_setopt( $curl, CURLOPT_POST, TRUE);
        curl_setopt( $curl, CURLOPT_POSTFIELDS, $postfields );
        curl_setopt( $curl, CURLOPT_USERAGENT, 'demo');
        curl_setopt( $curl, CURLOPT_TIMEOUT, 1 );
        $data = curl_exec( $curl );
        curl_close( $curl );
    }
}