Call Ajax my project slows me down

I have the following call ajax;

function notificacion_json(){
    /* Cargar notificaciones_respuesta_solicitudes  */

    var resultado = document.getElementById("span_numero_notificaciones");  
    var xmlhttp;

    if(window.XMLHttpRequest){
        xmlhttp = new XMLHttpRequest();
    }else{
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }
        xmlhttp.onreadystatechange = function(){                
            if(xmlhttp.readyState === 4 && xmlhttp.status === 200){
                resultado.innerHTML = xmlhttp.responseText;
                //alert(xmlhttp.responseText);
            }
        }
        xmlhttp.open("POST", "php/ajax/notificaciones.php", true);
        xmlhttp.setRequestHeader("content-type", "application/x-www-form-urlencoded");
        xmlhttp.send('notificacion_json=' + '1');       
}

This function I have it within a setInterval that is repeated every 2 seconds, to call the file notificaciones.php ;

if(isset($_POST['notificacion_json'])){
$usuario_solicitante = $_COOKIE['nombre_usuario'];
$carpeta_destino = $_SERVER[ 'DOCUMENT_ROOT' ] . "/intranet/uploads/perfiles/".$usuario_solicitante;
$nombre_archivo = "/metadata.json";
if(file_exists($carpeta_destino)){
    if (file_exists($carpeta_destino.$nombre_archivo)){
        $jsondata = fopen($carpeta_destino.$nombre_archivo,"r");
                //Si el archivo ya existe
                $jsondata = file_get_contents($carpeta_destino.$nombre_archivo); //Leer archivo
                $data = json_decode($jsondata, true); //Decodificandolo para convertirlo en un array asociativo
                echo $data[$usuario_solicitante]['notificaciones'];
        }
}

everything works perfect until a certain number of users connect (currently I have more than 150 simultaneous users) as soon as I implement this feature the web becomes extremely slow.

Is there an alternative to this or maybe I'm wrong with my code?

NOTE: The response size is 239b.

  

Possible Solution; Instead of being updated every 2 seconds I have placed it every 70 seconds, it would not be in real time but it works and does not overload the network.   I will study Node.js to make a system up to the standard.