Performance loss migration to VTK9 and Qt6

I am in the process of migrating software that was running on qt5 with QML (use of framebuffers …) and VTK 8.1 to Qt6 and VTK 9.1. VTK9 adds QQmlVTKPlugin for easy integration into qml code, so I use this solution to simplify my code, but after adding the same objects to my 3D scene as with the version on Qt5 and VTK8, I observe significant losses performance. Are there any known reasons for performance loss in my configuration? Could this come from the QQmlVTKPlugin or just from my implementation of this qml module?
Thanks for your help.