QTextCursor.insertText() extremely slow

#include <qapplication.h>
#include <qdialog.h>
#include <qlineedit.h>
#include <qstring.h>
#include <qtextcursor.h>
#include <qtextdocument.h>
#include <qtextedit.h>
#include <qpushbutton.h>
#include <qlayout.h>
#include <stdio.h>
#include <time.h>
#include <QDebug>

class cTextEditor : public QDialog {
  Q_OBJECT
private:
  QTextEdit *edit;
  QPushButton *b;
private slots:
  void ReplaceTest(void);
public:
  cTextEditor(void);
  };

cTextEditor::cTextEditor(void)
{
  QVBoxLayout *vbl = new QVBoxLayout(this);
  edit = new QTextEdit;
  vbl->addWidget(edit);
  b = new QPushButton("Replace Test");
  vbl->addWidget(b);
  connect(b, SIGNAL(clicked(bool)), SLOT(ReplaceTest()));
  resize(400, 200);
  QString qs;
  for (int i = 10000; --i > 0; )
      qs += "Layer 1;\nRect R0 (5.25 -4.75) (5.75 -4.25);\n";
  edit->setPlainText(qs);
}

void cTextEditor::ReplaceTest(void)
{
  QTextDocument *d = edit->document();
  if (d) {
     time_t t0 = time(NULL);
     QString FindText = "Layer 1;",
             Replace  = "Layer 94;";
     setUpdatesEnabled(false);
     QTextCursor tc;
     b->setDisabled(true);
     for (;;) {
         tc = d->find(FindText, tc, QTextDocument::FindCaseSensitively);
         if (tc.isNull())
            break;
         tc.insertText(Replace);
         edit->setTextCursor(tc);

         }
     b->setDisabled(false);
     setUpdatesEnabled(true);
     fprintf(stderr, "time = %ds\n", time(NULL) - t0);
     }
}

int main(int argc, char **argv)
{
  QApplication a(argc, argv);
  QDialog *d = new cTextEditor;
  d->show();
  return a.exec();
}

#include "main.moc"