Transition of an app Qt 4.x to Qt 5 - QTreeView problem

[Qiieha]

Hi,
I developed an application, which uses QTreeView (Model-View).
If I compile the same application on same machine with Qt 4.8 the performance of QTreeView is great, but if I compile the application with Qt 5.1 the performance is very very bad.

Example: Open QTreeView with 15000 rows takes 3 secs with Qt 4.8.
Compiled with Qt 5.1 it takes 10 times longer.

What can be the reason. The model is programmed correctly with lots of optimizing.

It is really annoying, because i invested lots of time in transition. It is a complex application.

Edit: I tested this on linux(gcc) and windows(mingw). The behaviour is the same.

thank you