Java Swing Games perform poorly when many sprites are on the screen

I'm making a simple tower defense game in swing. When I try to place many elves (more than 20) on the screen, I encounter a performance problem

The whole game takes place on a JPanel with setignorerepair (true) This is the paintcomponent method (con is the controller):

public void paintComponent(Graphics g){
    super.paintComponent(g);
    //Draw grid
    g.drawImage(background,null);
    if (con != null){
        //Draw towers
        for (Tower t : con.getTowerList()){
            t.paintTower(g);
        }
        //Draw targets
        if (con.getTargets().size() != 0){
            for (Target t : con.getTargets()){
                t.paintTarget(g);
            }
            //Draw shots
            for (Shot s : con.getShots()){
                s.paintShot(g);
            }
        }
    }
}

The target class simple draws bufferedimage at its current position The getimage method does not create a new bufferedimage, but only returns an instance of the controller class:

public void paintTarget(Graphics g){
    g.drawImage(con.getImage("target"),getPosition().x - 20,getPosition().y - 20,null);
}

Each target runs a swing timer to calculate its position This is the actionlistener it calls:

public void actionPerformed(ActionEvent e) {
    if (!waypointReached()){
        x += dx;
        y += dy;
        con.repaintArea((int)x - 25,(int)y - 25,50,50);
    }
    else{
        moving = false;
        mover.stop();
    }
}

private boolean waypointReached(){
    return Math.abs(x - currentWaypoint.x) <= speed && Math.abs(y - currentWaypoint.y) <= speed;
}

In addition, repaint() is called only when a new tower is placed

How to improve performance?

Solution

This may be your problem – let each target / bullet (I assume?) Being responsible for tracking when to update yourself and drawing yourself sounds like a lot of work A more common method is to cycle along the line

while (gameIsRunning) {
  int timeElapsed = timeSinceLastUpdate();
  for (GameEntity e : entities) {
    e.update(timeElapsed);
  }
  render(); // or simply repaint in your case,I guess
  Thread.sleep(???); // You don't want to do this on the main Swing (EDT) thread though
}

Essentially, an object in the chain is responsible for tracking all entities in the game and telling them to update and render them