S.Onur Selamet 
In some articles they mention increasing the graphic performance of QT by setting environment variable DFBARGS to no-hardware. You can add export DFBARGS=no-hardware to your /etc/init.d/rcS file or run scripts of your applications. I believe i got a little bit better performance too but not sure actually. I know there are many developers having unsatisfactory graphics performance. I hope this might help them. Can you please try and share your results here?