First delay in web API call from Android app

I have an Asp.Net Web API that can receive an HTTP request. I tested a web service from Postman - Rest Client or some other similar application and everything works great: the web service responds instantly to me. But if I try to call the same web service from my Android app I have a problem:

The first call to the web service is slow and the next is fast. If I stop calling ws and then repeat, for example after 1 minute, the ws response is slow again, and the next ones are fast ... and so on.

Here's the code I am using for the web service call.

String wsURI = "www.myWsUrl...";
url = new URL(wsURI);

try{    
    httpConnection = (HttpURLConnection) url.openConnection();
    httpConnection.setDoInput(true); // ?
    httpConnection.setDoOutput(true);
    httpConnection.setRequestMethod("POST");
    httpConnection.setRequestProperty("Content-Type", "application/json");

    JSONStringer requestData = new JSONStringer()
         .object()
              .key("id").value(12)
              .key("frequence").value(1000)
              .key("code").value("ABCAB-0123")
                  .endObject();

    BufferedWriter out = new BufferedWriter(new  OutputStreamWriter(httpConnection.getOutputStream()));
    out.write(requestData.toString());
    out.close();

    resCode = httpConnection.getResponseCode();
...

    }catch(){
        ...
    }finally{
        if(httpConnection!=null)
            httpConnection.disconnect();
    }