We have had some reports of this freezing on and off, but only incidentally and have not found a way to reproduce it. Nor does it happen continuously with the few other reports that we had. If it happens with you consistently with every session, could you share your system configuration and development workflow through a support ticket?
That might give us some concrete starting points to investigate what could be causing this freezing.
We have been running a private and then public beta for four months with Mendix 9, so the product in general is very stable. However there are always edge cases or specific development environments that could be experiencing problems and we would definitely want to solve those too.