QSyntaxHighlighter hangs indefinitely on rehighlight function for large text files > 10mb

On older versions < 4 of Qt the rehighlight function works fine for large files. However on 4.7.4 on a large text file > 10mb, rehighlight hangs indefinitely. I tried version 4.5.3 and rehighlight is slow, but finishes in a couple seconds. To reproduce I added the slot:

void MainWindow::refresh()
{
QApplication::setOverrideCursor(QCursor(Qt::WaitCursor));
highlighter->rehighlight();
QApplication::restoreOverrideCursor();
}

to the syntaxHighlighter example and created a menu option for it. Then I opened a large text file ~15mb and called refresh.