Qt 3D performance issue after upgrading from 6.4.3 to 6.8.0

After upgrading Qt to 6.8.0 (from 6.4.3), our application became pratically unusable, scrolling through ListView, manipulating a model in a 3D view or displaying a popup is really slow.

To try understanding what is going on, I used the QSG_VISUALIZE=batches and compared both version of Qt, in 6.4.3 every colors are fixed, but in 6.8.0 it's constantly blinking. I don't know if this is normal.

I replicated the issue by modifying the simple-qml example from Qt 3D and attached a video (and the sources) to show you the issue, on the left 6.4.3 and on the right 6.8.0.

I also tried with Qt 6.5.3 and the issue was already there.

In our application we are using the OpenGL renderer backend, not RHI.