Scenario is lagging

public class game3 extends World
{
    private int spawnX = 0;
    private int spawnX2 = 0;
    private int spawnX3 = 0;
    private int spawnCounter = 0;
    private int spawnTreasure = 0;
    private Counter theCounter;
    private static final String bgImageName = "watergoed.jpg";
    private static final int scrollSpeed = 3;
    private static final int picHeight = (new GreenfootImage(bgImageName)).getHeight();
    private GreenfootImage bgImage, bgBase;
    private int scrollPosition = 0;
    private int timer = 3600;
    TimerText timerText = new TimerText();
    /**
     * Dit is de constructor voor game3. 
     * We pakken ook de background image hier zodat we in een andere methode 
     * de background kunnen tekenen zodat het lijkt alsof die naar beneden scrolled.
     */

    public game3()
    {    
        super(900, 900, 1); 
        setBackground(bgImageName);
        bgImage = new GreenfootImage(getBackground());
        bgBase = new GreenfootImage(getWidth(), picHeight);
        bgBase.drawImage(bgImage, 0, 0);
        prepare();
        addObject(timerText, 100, 15); //wherever
        timerText.setText("Time left: " + (timer/60));
    }
    /**
     * We returnen de waarde van theCounter hier.
     */
    public Counter getCounter()
    {
        return theCounter;
    }
    /**
     * In de act methode tekenen we de background image voor de scroll effect. 
     * We hebben ook de methode addObjects gebruikt om random objecten te spawnen in de wereld.
     * Ook de werking van de timer staat erin.
     */
    public void act()
    {
        addObjects();
        if (timer%60==0) timerText.setText("Time left: " + (timer/60));
         timerFunc();
        scrollPosition += scrollSpeed;
        while(scrollSpeed > 0 && scrollPosition > picHeight) scrollPosition -= picHeight;
        while(scrollSpeed < 0 && scrollPosition < 0) scrollPosition += picHeight;
        paint(scrollPosition);
    }
}
       
        
    /**
     * Hier word de background image echt getekend.
     */
    private void paint(int position)
    {
        GreenfootImage bg = getBackground();
        bg.drawImage(bgBase,0, position);
        bg.drawImage(bgImage, 0, position-(int)Math.signum(scrollSpeed)*picHeight);
    }