JavaFX TextField Append delay

So, I made a simple server-client chat program, but I have a problem:
if the server tries to message the client, the server lags or freezes a bit, same with the client.
is there something that I do wrong?

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;

import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.control.TextArea;
import javafx.scene.control.TextField;
import javafx.scene.paint.Color;
import javafx.stage.Stage;

public class Server extends Application {

    private Group serverGR;
    private Scene serverSC;
    private Stage window;
    private TextField tf;
    private TextArea ta;

    private ServerSocket ss;
    private Socket s;
    private DataInputStream dis;
    private DataOutputStream dos;

    ConnectionThread cT = new ConnectionThread();

    public void init() {
        cT.start();
    }

    @Override
    public void start(Stage primaryStage) {
        try {
        serverGR = new Group();
        serverSC = new Scene(serverGR, 800, 500, Color.LIGHTCYAN);
        window = primaryStage;
        window.setTitle("Bleidorb Simplistic ChatProgram \\\\\\\\\"ServerMode\"\\\\\\\\");
        window.setScene(serverSC);
        window.show();

        tf = new TextField();
        tf.setTranslateX(10);
        tf.setTranslateY(10);
        tf.setPrefWidth(400);
        tf.setPrefHeight(20);
        tf.appendText("Type your message here.");
        tf.setOnAction(event -> {

            String server = "Server: ";
            server += tf.getText();

                try {
                    dos.writeUTF(server);
                    dos.flush();
                    ta.appendText(dis.readUTF() + "\n");
                } catch (IOException e) {
                    e.printStackTrace();
                }

            tf.clear();

        });

        ta = new TextArea();
        ta.setTranslateX(10);
        ta.setTranslateY(40);
        ta.setPrefWidth(400);
        ta.setPrefHeight(400);

        serverGR.getChildren().addAll(tf, ta);
        } catch (Exception e) {
            System.out.println(e.toString() + "\n something wrong in startmethod, lol?");
        }
    }   

    @SuppressWarnings("deprecation")
    public void stop() {
        try {
            cT.stop();
            dis.close();
            ss.close();
            s.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        launch(args);
    }

    private class ConnectionThread extends Thread {

        @Override
        public void run(){
            try {
                ss = new ServerSocket(5279, 100);
                s = ss.accept();
                s.setTcpNoDelay(true);
                dis = new DataInputStream(s.getInputStream());
                dos = new DataOutputStream(s.getOutputStream());
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.Socket;

import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.control.TextArea;
import javafx.scene.control.TextField;
import javafx.scene.paint.Color;
import javafx.stage.Stage;

public class Client extends Application {

    private Group serverGR;
    private Scene serverSC;
    private Stage window;
    private TextField tf;
    private TextArea ta;

    private Socket s;
    private DataInputStream dis;
    private DataOutputStream dos;

    ConnectionThread cT = new ConnectionThread();

    public void init() {
        cT.start();
    }

    @Override
    public void start(Stage primaryStage) {
        try {
        serverGR = new Group();
        serverSC = new Scene(serverGR, 800, 500, Color.LIGHTCYAN);
        window = primaryStage;
        window.setTitle("Bleidorb Simplistic ChatProgram \\\\\\\\\"ClientMode\"\\\\\\\\");
        window.setScene(serverSC);
        window.show();

        tf = new TextField();
        tf.setTranslateX(10);
        tf.setTranslateY(10);
        tf.setPrefWidth(400);
        tf.setPrefHeight(20);
        tf.appendText("Type your message here.");
        tf.setOnAction(event -> {

            String client = "Client: ";
            client += tf.getText();

                try {
                    dos.writeUTF(client);
                    dos.flush();
                    ta.appendText(dis.readUTF() + "\n");
                } catch (IOException e) {
                    e.printStackTrace();
                }

            tf.clear(); 

        });

        ta = new TextArea();
        ta.setTranslateX(10);
        ta.setTranslateY(40);
        ta.setPrefWidth(400);
        ta.setPrefHeight(400);

        serverGR.getChildren().addAll(tf, ta);
        } catch (Exception e) {
            System.out.println(e.toString() + "\n something wrong in startmethod, lol?");
        }
    }   

    @SuppressWarnings("deprecation")
    public void stop() {
        try {
            cT.stop();
            dos.close();
            s.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        launch(args);
    }

    private class ConnectionThread extends Thread {

        @Override
        public void run(){
            try {
                s = new Socket("localhost", 5279);
                s.setTcpNoDelay(true);
                dis = new DataInputStream(s.getInputStream());
                dos = new DataOutputStream(s.getOutputStream());
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}