Paralelize downloads in order to increase speed over HTTP 1.1

paralelize_downloads.php

<?php

/**
 * Paralelize download test
 */

add_filter( 'wp_get_attachment_url', 'iv_parallelize_hostnames', 10, 2 );

function iv_parallelize_hostnames( $url, $id ) {
	$hostname = iv_get_hostname( $url );
	$url      = str_replace( parse_url( get_bloginfo( 'url' ), PHP_URL_HOST ), $hostname, $url);

	return $url;
}

function iv_get_hostname( $name ) {
	$subdomains = array(
		'www.domain.com',
		'ww1.domain.com',
		'ww2.domain.com',
		'ww3.domain.com',
	);

	$host     = abs( crc32( basename( $name ) ) % count( $subdomains ) );
	$hostname = $subdomains[ $host ];

	return $hostname;
}