What is causing a very slow ajax response?

Question

I wrote some PHP code to help me connect to a REST API for a telephone system (ie. ICWS.php.)

Then to make my life easier, I wrote a small script (ie. interations.php) that accepts two parameters: a method and an ID. This script will basically call a public method in my PHP connector.

In addition, I have another script (ie. poll.php). This script will ping the API once every half a second to see if there is a new message available. I am using server-side polling to handle this. The code below will show how poll.php

    while(1){
        //process Messages
        $icws->processMessages();
        //show the Calls Queue
        $result = $icws->getCallsQueue();
        //$current = $icws->getCurrentUserStatusQueue();
        echo 'event: getMessagingQueue' . "\n";
        echo 'data: ' . json_encode( array('calls' => $result));    
        echo "\n\n"; //required  

        ob_flush();
        flush(); 
        putToSleep($sleepTime);

    }
function putToSleep($val = 1){

    if( is_int($val) ){
        sleep($val);
    } else {
        usleep($val * 1000000);
    }
}

From my site (ie. phonesystem.html) I start server-side polling "which pings the API once every 1/2 seconds." From the same page, I can also make other direct calls (ie. Dial 7204536695); all requests are done via Ajax.

Here is my code that generates the server-side polling

    //Server Side Message Polling
    function startPolling(){

        var evtSource = new EventSource("poll.php");


        evtSource.addEventListener("getMessagingQueue", function(e) {
        var obj = JSON.parse(e.data);

          if(!obj.calls || obj.calls.length === 0){
              console.log('no messages');
              phoneKeyPad(false);
              return;
          }
          processMessages(obj.calls);

        }, false);
    }


    $(function(){
         startPolling();
    });

The problem that I am facing is that when making an ajax call, the response takes way too long (+1 minute).

It seems that the Apache server slows down as using other application becomes a little slower.

What can I check and how can I trouble shoot this problem?