Node-JS handle multiple request simultaneously

I created a server which listen to port 8888:

    http.createServer(function (req, res) {
        res.writeHead(200, {'Content-Type': 'text/html'});
        console.log('start server');

        req.on('data', function (data) {
            var 
                json = null,
                url_parts = {};

            url_parts = url.parse(req.url);
            json = JSON.parse(data);
            if (Router.route(url_parts, json, res) === false) {
                console.log('error with routing');
                res.end('error with routing');
                return;
            }
            console.log('end of request');
            res.end('success');
        });
    }).listen(8888);

In Router.route, there's an heavy code executing.

The problem is-when someone access the url http://localhost:8888/SOME_URLanother calling to this port like: http://localhost:8888/SOME_OTHER_URL he has to wait a lot of time..

How can I handle both calls simultaneously?

Tnx for your help!