In our code editing software (https://github.com/aseba-community/aseba), the editor is inherited from QTextEdit. The GUI is single threaded.
With Qt 4.8, the widget's refresh is considerably slowed down on Windows when typing new characters (tested on Windows XP and Seven 64-bits). This was not experienced with Qt 4.7.4.
After bisecting, the problem appears with the following commit:
Trond Kjernåsen <email@example.com> 2010-04-07 14:57:24
Make it possible to call QWidget::metric() from a different thread.
Reverting the commit in Qt 4.8 solves the issue.