Increase of parallel requests form node.js to external system takes more time to respond

I have a simple case where I'm requesting a different upstream proxy server from my node.js server. With the increase in load I see the request takes lot of time to execute(though time taken to respond from my upstream proxy server is constant across the requests). To demonstrate the issue i've written a sample program as below. If you execute the below program, the first request takes 118ms to execute and the last one takes 10970ms. If you observe i'm using async to parallelize my requests.

The question is, what is the reason node.js takes this much time to execute a request when run in parallel. To give some more context on the infra settings(centos 6.5) I have opened up my port range from 1024 to 65535, change the fin_timeout to 15 seconds and enable tw_reuse =1 for sockets in sysctl.conf

var http = require('http');
var uuid = require('node-uuid');
var async = require('async');

function callExternalUrl(){
    var uniqueId = uuid.v4();
    console.time(uniqueId);
    var options = {
        host: 'google.com',
        port: '80',
        path: '/',
        method: 'GET'
    };
    var req = http.request(options, function(res) {
        var msg = '';
        res.setEncoding('utf8');
        res.on('data', function(chunk) {
            msg += chunk;
            console.timeEnd(uniqueId);
        });
        res.on('end', function() {
        });
    });
    req.end();
}

function iterateAsync(callback){
    var iter = [];
    for(var i=0; i<1000; i++){
        iter[i] = i;
    }
    async.each(iter,
        function(item, callback) {
            callExternalUrl();
        },
        function(err) {
            callback(err);
        }
    );
}

iterateAsync(function(){console.log('done');});