Creating Game using multithreading, but have problems with smoothness of background movement in Java Swing

Question

I'm working on a project which is about creating a game. I was implementing threads to move my background but had some problems with the smoothness of background movement. I also have a bird component, but it is supposed to be used with keybindings. Both of these components are drawn by using the paintComponent() method.

The problem is when I start the game, the background starts moving, but after 1-2 seconds, it starts lagging, but when I use keys to move my bird component right and left, the background movement becomes smooth again, and when I don't press any key, it starts lagging again. I also observed that when I hover the mouse over the window, it also starts moving smoothly. I couldn't realize the problem. Tried to use alternatives instead of original ones, like using labels instead of paintComponent() and using timer. instead of thread, but nothing changes.

I also searched about this problem a lot, and found some options like PC performance and so on. But I don't think that I have a problem with that. Is there a way to solve this problem? Thanks beforehand! Here is the code:

FlyingChicken.java

This is the main class to call other classes

import java.io.*;
import java.time.*;
import java.lang.*;
import java.util.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.Timer;

public class FlyingChicken extends JFrame implements ActionListener{

    JButton startButton, firstLoginButton, firstRegisterButton, firstQuitButton;
    boolean gameStarted = false, gameOver = false, gamePaused = false;
    RegisterPage registerPage;
    LoginPage loginPage;
    GamePage gamePage;


    public FlyingChicken(){

        //FirstRegisterButton
        firstRegisterButton = new JButton("Register Account");
        firstRegisterButton.setBounds(360,445,200,50);
        firstRegisterButton.setFocusable(false);
        firstRegisterButton.addActionListener(this);
        firstRegisterButton.setBackground(Color.green);
        firstRegisterButton.setForeground(Color.white);

        //FirstLoginButton
        firstLoginButton = new JButton("Login Account");
        firstLoginButton.setBounds(360,520,200,50);
        firstLoginButton.setFocusable(false);
        firstLoginButton.addActionListener(this);
        firstLoginButton.setBackground(Color.green);
        firstLoginButton.setForeground(Color.white);

        //FirstQuitButton
        firstQuitButton = new JButton("Quit the Game");
        firstQuitButton.setBounds(385,595,150,50);
        firstQuitButton.setFocusable(false);
        firstQuitButton.addActionListener(this);
        firstQuitButton.setBackground(Color.green);
        firstQuitButton.setForeground(Color.white);

        //StartButton
        startButton = new JButton("Start Game");
        startButton.setBounds(360,495,200,50);
        startButton.setFocusable(false);
        startButton.addActionListener(this);
        startButton.setVisible(false);
        startButton.setFont(new Font("Arial",Font.BOLD, 20));

        loginPage = new LoginPage();
        registerPage = new RegisterPage();
        gamePage = new GamePage();

        loginPage.loginPageVisibility(false);
        registerPage.registerPageVisibility(false);
        gamePage.gamePageVisibility(false);

        loginPage.backButton.addActionListener(this);
        loginPage.loginButton.addActionListener(this);
        registerPage.registerButton.addActionListener(this);
        registerPage.backButton.addActionListener(this);
        gamePage.quitButton.addActionListener(this);

        //Frame Settings
        Image img = new ImageIcon("Flying_Chicken_Icon.png").getImage();
        this.setIconImage(img);
        this.setSize(920,1080);
        this.setTitle("Flying Chicken");
        this.getContentPane().setBackground(new Color(14, 194, 176));
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setResizable(false);
        this.setFocusable(true);
        this.setLayout(null);
        this.setVisible(true);
        this.add(loginPage);this.add(registerPage);
        this.add(gamePage);this.add(startButton);
        this.add(firstQuitButton);this.add(firstRegisterButton);
        this.add(firstLoginButton);

    }

    public void mainPanel(){
        this.getContentPane().setBackground(new Color(14, 194, 176));
        firstRegisterButton.setVisible(true);
        firstQuitButton.setVisible(true);   
        firstLoginButton.setVisible(true);
        loginPage.loginPageVisibility(false);
        registerPage.registerPageVisibility(false);
        loginPage.loginLabel.setVisible(false);
        registerPage.label1.setVisible(false);
        registerPage.label2.setVisible(false);
        registerPage.label3.setVisible(false);
        registerPage.label4.setVisible(false);
        registerPage.label5.setVisible(false);  
    }

    public void loginPanel(){

        loginPage.tpassword.setText("");
        loginPage.tusername.setText("");
        firstRegisterButton.setVisible(false);
        firstQuitButton.setVisible(false);  
        firstLoginButton.setVisible(false);
        loginPage.loginPageVisibility(true);
        registerPage.registerPageVisibility(false);
        
    }

    public void registerPanel(){

        loginPage.loginPageVisibility(false);
        registerPage.registerPageVisibility(true);
        registerPage.tpassword.setText("");
        registerPage.tusername.setText(""); 
        firstRegisterButton.setVisible(false);
        firstQuitButton.setVisible(false);  
        firstLoginButton.setVisible(false);
    }

    public void startPage(){

        this.getContentPane().setBackground(new Color(128,198,209));
        startButton.setVisible(true);
        loginPage.loginPageVisibility(false);
        registerPage.registerPageVisibility(false); 
    }

    public void startGame(){

        startButton.setVisible(false);
        gamePage.gamePageVisibility(true);
        // gamePage.timer.start();
        gameStarted = true;
    }

    public void quitFunction(){
        String[] resp = {"Resume", "Quit"};
        int k = JOptionPane.showOptionDialog(this, "Are you sure you want to quit?","Quit Panel",JOptionPane.YES_NO_OPTION,JOptionPane.INFORMATION_MESSAGE, null, resp, 0);
        if (k==1){
            this.setVisible(false);
            this.dispose();
        }
    }

    @Override
    public void actionPerformed(ActionEvent e){
        if(e.getSource() == startButton){
            startGame();
        }
        if(e.getSource() == gamePage.quitButton){
            // gamePage.timer.stop();
            quitFunction();
        }
        if(e.getSource() == firstQuitButton){
            quitFunction();
        }
        if(e.getSource() == firstRegisterButton){
            registerPanel();
        }
        if(e.getSource() == firstLoginButton){
            loginPanel();
        }
        if(e.getSource() == registerPage.registerButton){
            if(registerPage.registerValidate() == false){
                registerPage.registerData();
                loginPanel();
            }
        }
        if(e.getSource() == loginPage.loginButton){
            if(loginPage.loginValidate() == false)
                startPage();
        }
        if(e.getSource() == loginPage.backButton || e.getSource() == registerPage.backButton){
            mainPanel();
        }
    }

    public static void main(String[] args){

        FlyingChicken game = new FlyingChicken();
    }
}

GamePage.java

This is the class that I'm supposed to create animations and game

import java.io.*;
import java.time.*;
import java.lang.*;
import java.util.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.Timer;

public class GamePage extends JPanel implements ActionListener, Runnable{

    JLabel scoreBoard;
    Image backgroundImage, birdImage, catImage;
    JButton quitButton;
    int birdX, birdY, xpos=0, ypos=0, score = 0;
    Timer timer;
    Thread animator;
    Action leftAction, rightAction;

    public GamePage(){

        //GamePage Panel Settings
        this.setSize(920,1080);
        this.setLocation(xpos,ypos);
        this.setLayout(null);
        this.setFocusable(true);
        this.setVisible(true);

        backgroundImage = new ImageIcon("background.jpg").getImage();
        birdImage = new ImageIcon("bird.png").getImage();
        birdX = 264;birdY = 230;



        //Scoreboard
        scoreBoard = new JLabel("Score: "+ score);
        scoreBoard.setOpaque(false);
        scoreBoard.setForeground(new Color(33,30,31));
        scoreBoard.setFont(new Font("Arial", Font.BOLD, 25));
        scoreBoard.setBounds(0,0,200,30);
        scoreBoard.setVisible(true);

        //QuitButton
        quitButton = new JButton("Quit");
        quitButton.setBounds(820,0,100,50);
        quitButton.setFocusable(false);
        quitButton.setVisible(true);

        // timer = new Timer(10,this);
        createKeyBindings(this);

        this.add(quitButton);
        this.add(scoreBoard);
    }

    public void addNotify(){
        super.addNotify();
        animator = new Thread(this);
        animator.start();
    }

    @Override
    public void run(){

        while(true){
            try{
                ypos = ypos-1;
                repaint();
                Thread.sleep(25);
            } catch(InterruptedException e){System.out.println("hey");}         
        }
    }

    //The function to draw background
    @Override
    public void paintComponent(Graphics g){

        super.paintComponent(g);
        g.drawImage(backgroundImage, xpos, ypos, null);
        g.drawImage(birdImage,birdX,birdY,null);
    }

    @Override
    public void actionPerformed(ActionEvent e){
        
        // scoreBoard.setText("Score: "+score);
        repaint();
    }

    //The function to create keybindings
    public void createKeyBindings(JPanel p) {

        InputMap im = p.getInputMap(JPanel.WHEN_IN_FOCUSED_WINDOW);
        ActionMap am = p.getActionMap();
        leftAction = new LeftAction();
        rightAction = new RightAction();
        im.put(KeyStroke.getKeyStroke("LEFT"), "left");
        im.put(KeyStroke.getKeyStroke("RIGHT"), "right");
        am.put("left", leftAction);
        am.put("right", rightAction);
    }

    public class LeftAction extends AbstractAction{
        @Override
        public void actionPerformed(ActionEvent e){
            if(birdX>0)
                birdX = birdX-10;
            repaint();
        }
    }

    public class RightAction extends AbstractAction{
        @Override
        public void actionPerformed(ActionEvent e){
            if(birdX+530<1080)
                birdX = birdX+10;
            repaint();
        }
    }

    public void gamePageVisibility(boolean x){
        this.setVisible(x);
    }

}

EDIT: I found the actual reason my application was lagging. The problem was mainly related with Linux OS graphics and solved it by adding Toolkit.getDefaultToolkit().sync() to my loop.