Http Request can't send a response immediately in Nodejs server

I send JSON requests one by one to the nodejs server. After 6th request, server can't reply to the client immediately and then it takes a little while(15 seconds or little bit more and send back to me answer 200 ok) It occurs a writing json value into MongoDB and time is important option for me in terms with REST call. How can I find the error in this case? (which tool or script code can help me?) My server side code is like that

var controlPathDatabaseSave = "/save";
app.use('/', function(req, res) {
console.log("req body app use", req.body);
var str= req.path;

if(str.localeCompare(controlPathDatabaseSave) == 0)
{
    console.log("controlPathDatabaseSave");
    mongoDbHandleSave(req.body);
    res.setHeader('Content-Type', 'application/json');
    res.write('Message taken: \n');
    res.write('Everything all right with database saving');
    res.send("OK");
    console.log("response body", res.body);
}

});

My client side code as below:

function saveDatabaseData()
{
    console.log("saveDatabaseData");
        var oReq = new XMLHttpRequest();
        oReq.open("POST", "http://192.168.80.143:2800/save", true);
        oReq.setRequestHeader("Content-type", "application/json;charset=UTF-8");
        oReq.onreadystatechange = function() {//Call a function when the state changes.
            if(oReq.readyState == 4 && oReq.status == 200) {
                console.log("http responseText", oReq.responseText);
            }
        }
        oReq.send(JSON.stringify({links: links, nodes: nodes}));
}

You can see my REST call in google chrome developer editor. (First six call has 200 ok. Last one is in pending state)

Thanks in advance,

via zoint