Why is the Document.insertString () runtime not constant?

Question

Why is the Document.insertString () runtime not constant?

I am working on a registrar to show the result as part of the larger Java swing GUI. Unfortunately, after adding, I experienced a slowdown. I tracked the issue for repeated calls Document.insertString().

I did a test that shows this slowdown:

LogPanel.java

public class LogPanel extends JPanel{
    private static final SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    private JEditorPane textPane;
    private static int numTextRows = 50;
    private SimpleAttributeSet keyWord;
    private Document document;

   public LogPanel() {
        super(new BorderLayout());
        add(makePanel(), BorderLayout.CENTER);
    }

    private Component makePanel() {
        // Just a text area that grows and can be scrolled.
        textPane = new JTextPane();
        document = textPane.getDocument();
        keyWord = new SimpleAttributeSet();
        StyleConstants.setForeground(keyWord, Color.BLACK);

        //textArea.setRows(numTextRows);
        textPane.setEditable(false);
        textPane.setFont(new Font("monospaced", Font.PLAIN, 12));

        DefaultCaret caret = (DefaultCaret) textPane.getCaret();
        caret.setUpdatePolicy(DefaultCaret.ALWAYS_UPDATE);

        //Wrap the textPane in a JPanel with BorderLayout so that the text does not wrap
        JPanel textPaneWrapper = new JPanel();
        textPaneWrapper.setLayout(new BorderLayout());
        textPaneWrapper.add(textPane);

        JScrollPane areaScrollPane = new JScrollPane(textPaneWrapper);
        areaScrollPane.getVerticalScrollBar().setUnitIncrement(20);
        //JScrollPane areaScrollPane = new JScrollPane(textPane);
        areaScrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
        areaScrollPane.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
        areaScrollPane.setPreferredSize(new Dimension(250, 250));
//        builder.add(areaScrollPane, cc.xywh(1, 3, 3, 1));

        StyleConstants.setBackground(keyWord, areaScrollPane.getBackground());
        textPane.setBackground(areaScrollPane.getBackground());

        return areaScrollPane;
    }

    public void appendResult(final String action, final String result, final Color color) {
        Date now = new Date();
        String strDate = df.format(now);
        String paddedAction = String.format("%-19s", action);
        StyleConstants.setForeground(keyWord, color);

        try {
            document.insertString(document.getLength(), strDate + "   " + paddedAction + "   " + result + "\n", keyWord);
        } catch (BadLocationException e) {
            throw new RuntimeException(e);
        } 

        if(!textPane.hasFocus()) {
            textPane.setCaretPosition(document.getLength());
        }
    }

    public void appendResult(String action, String result) {
        appendResult(action, result, Color.BLACK);
    }
}

LogTester.Java

public class LogTester extends JFrame{
    private LogPanel logPanel;
    private JButton pushMe;

    public LogTester(){
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new GridBagLayout());

        logPanel = new LogPanel();
        this.add(logPanel);

        pushMe= new JButton("Press Me");
        LogTester self=this;
        pushMe.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e) {
                self.addLotsOfStuff();
            }
        });
        this.add(pushMe);

        this.pack();
        this.setVisible(true);

    }

    public void addLotsOfStuff(){
        for(int i=0; i<1000; i+=1){
            long start=System.currentTimeMillis();
            for(int j=0; j<1000; j+=1){
                String str="This is a very long peice of text designed to test the capabilites of our log panel. Move along, nothing to see here.";
                logPanel.appendResult("HERP",str);
            }
            long end=System.currentTimeMillis();
            System.out.println(end-start);
        }
    }

    public static void main(String args[]){
        LogTester test=new LogTester();
    }
}

The program above tries to write a large number of lines to JTextPanewhere it is used Document.insertString(). The results of this program relate to: