Many Turtles & Arc => flickering

Tu16.java

// Tu16java

package app.tu16;

import turtle.*;

public class Tu16 extends Playground
{
  public void playgroundPressed(double x, double y)
  {
    Turtle joe = new Turtle(x, y);
    arc(joe);

  }
  
  	private void arc(Turtle t) {
		for (int i = 0; i < 720 / 3; i++) {
			t.fd(1);
			t.rt(3);
		}
	}
}

When calling fd(..) in short succession with many turtles on the board, they start to flicker. Issue does not show in http://www.android-turtlegrafik.ch/index.php?inhalt_links=navigation.inc.php&inhalt_mitte=tu/events.inc.php