Java Swing - 如何重画JFrame滞后/弹出

import java.awt.Color;
import java.awt.Graphics;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import java.util.Random;

import javax.swing.Icon;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.Timer;

public class Main extends JPanel {
  static Color[] COLORS = { Color.blue, Color.red, Color.yellow,
      Color.orange, Color.green, Color.cyan, Color.lightGray, Color.magenta,
      Color.white, Color.black };
  static Icon[] ICONS = new Icon[COLORS.length];
  static int ROWS = 20;
  static int COLS = 30;
  static int BI_WIDTH = 20;
  static int BI_HEIGHT = BI_WIDTH;
  static int TIMER_DELAY = 15;
  static int NUMBER_TO_SWAP = 15;
  JLabel[][] grid = new JLabel[ROWS][COLS];
  Random random = new Random();
  static {
    for (int i = 0; i < ICONS.length; i++) {
      BufferedImage img = new BufferedImage(BI_WIDTH, BI_HEIGHT,
          BufferedImage.TYPE_INT_ARGB);
      Graphics g = img.getGraphics();
      g.setColor(COLORS[i]);
      g.fillRect(0, 0, BI_WIDTH, BI_HEIGHT);
      g.dispose();
      ICONS[i] = new ImageIcon(img);
    }
  }
  public Main() {
    setLayout(new GridLayout(ROWS, COLS));
    for (int row = 0; row < grid.length; row++) {
      for (int col = 0; col < grid[row].length; col++) {
        JLabel label = new JLabel();
        int index = random.nextInt(COLORS.length);
        label.setIcon(ICONS[index]);
        add(label);
        grid[row][col] = label;
      }
    }
    new Timer(TIMER_DELAY, new ActionListener() {
      @Override
      public void actionPerformed(ActionEvent evt) {
        for (int i = 0; i < NUMBER_TO_SWAP; i++) {
          int row = random.nextInt(ROWS);
          int col = random.nextInt(COLS);
          int iconIndex = random.nextInt(ICONS.length);
          grid[row][col].setIcon(ICONS[iconIndex]);
        }
      }
    }).start();
  }
  public static void main(String[] args) {
    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().add(new Main());
    frame.pack();
    frame.setVisible(true);
  }
}