Long TTFB from FB Graph API - php

Question

I wrote a PHP script home-made in order to get last FB posts from a page, which is simply executed from an jQuery/AJAX code when my document is ready :
$("my_object").load("fb_feed.php");
Running on local PHP server as on a VPS, to Time To First Byte is very long : 10s at least, 20s on average, or more
ini_set("allow_url_fopen", 1);
function getGraphUrl($param){
return "https://graph.facebook.com/". $param ."access_token=THE_TOKEN";
}
function getContent($param){
$url = getGraphUrl($param);
return json_decode(file_get_contents($url));
}
$feed = getContent("ID_OF_THE_PAGE/feed?");
$posts = array();
foreach ($feed->data as $i => $post) {
if(isset($post->message)){
$object_id = getContent($post->id . "?fields=object_id&")->object_id;
if(isset($object_id)){
$img_url = "https://graph.facebook.com/" . $object_id . "/picture";
$short = (getimagesize($img_url)[1] < 170 ? true : false);
if($short){
$img_url = "images/empty.png";
}
array_push($posts, array('id' => $post->id,'message' => $post->message,
'img_url' => $img_url, 'date' => $post->created_time));
}
}
if(count($posts) == 6){
break;
}
}
The extract of the PHP script which load posts 👆
Everything works, the only problem is the loading duration which is very very long
Is there a way to load faster, rather than build my own cache ?
Is my request poorly made ?