PHP REST Service is really slow

I'm having a problem with my PHP REST service, it is really slow. I build it with the Flight PHP Framework. It's accessing data from a MySQL database and returns them as json.

Flight REST Service:

Flight::route('GET /categories', function(){
    header('Content-type: application/json');
    $db = Flight::db();
    $sql = "SELECT * FROM categories";
    $stmt = $db->prepare($sql);
    $stmt->execute();
    $result = $stmt->fetchAll();
    echo json_encode($result);
    $db = null;
});

The first response is very fast and the answer is returned not formatted.

After ~3 seconds the request is finished and the answer is displayed correctly.

Why does it takes so long for finishing the request?

Thanks in advance!

Finally I fixed that behavior with an exit(); call at the end of the method:

Flight::route('GET /categories', function(){
   header('Content-type: application/json');
   $db = Flight::db();
   $sql = "SELECT * FROM categories";
   $stmt = $db->prepare($sql);
   $stmt->execute();
   $result = $stmt->fetchAll();
   echo json_encode($result);
   $db = null;
   exit();
});

It takes between 50-80ms for finishing the request now.