what is causing the performance drain is hard to tell without knowing more about your game.
The first step would be to start QML profiler and test on which parts of your game code the most time is spent. Alternatively, you can try by deactivating specific functionality where you think most processing time is spent. Or add a toggle button which allows changing this behavior at runtime where you then see the effects immediatelly.
Also, you can have a look if this performance issues go away in release mode which usually runs a lot faster.