Parallelize HttpsURLConnection in Java

Question

I am working on an application which executes a lot of HTTP-Post Request on a specific URL. The data gets included into the request via a JSON File. Performing a single POST request takes about 200ms until I get the response code back. In order to parallelize these Request I started using different Worker Threads, opening a HTTPURLConnection each and posting their requests over this connection. Unfortunately I do not see much improvement, so I wanted to clarify the underlying situation of the TCP sockets and get some thoughts from you guys.

Each of my workers executes the request like the following example could.

    public void submitRequest() {
    URL url = new URL("https://example.com");
    HttpsURLConnection conn = (HttpsURLConnection) url.openConnection();
    conn.setRequestProperty("Content-Type", "application/json");
    conn.setRequestProperty("Accept", "application/json");
    conn.setDoOutput(true);
    conn.setRequestMethod("POST");

    DataOutputStream output = new DataOutputStream(conn.getOutputStream());

    // Construct the POST data.
    String content = generatedJSONString();

    // Send the request data
    output.writeBytes(content);
    output.flush();
    output.close();

    // Check response code here
    int response = conn.getResponseCode();

}

Because each worker uses its own HttpsURLConnection, I assume that the necessary TLS handshake is only done once per worker and the requests can then be executed without the need of an additional handshake? I want each worker to use its own TCP connection to the server. I tried to check with Wireshark, but it does not show me the TLS handshake, I don't really know why.

Even with 50 workers in parallel, my code does not execute fast enough to post the relevant data to the url. Therefore I ask myself the question if its because of the TCP connections not getting saturated by the code, because the JSON data (usually 10 key-value pairs) is to little data compared to the TCP/HTTP overhead. If not I might ask the service provider to think about a better way to deliver data in a batch way, to submit more JSON data per individual POST request. I also changed to code to use the Apache HTTPClient, but I got similar performance, therefore I think there is a mistake in the way I think the TLS connections get established and reused.