When I should use QGraphicsView and When QtQuick is a better choice?
Which is better choice if I want high performance application with highly customized widgets (visual of widgets and its behaviors)?
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For high performances, you should use Qt and C++ instead of QML and JavaScript. So let's use a QGraphicsView in this case.
However, creating a widget is much more easier and faster with QML than QtWidget. It may be great for making prototypes or for interfaces which are made by designers. QML is a convinient solution if you do not need high performances. You can also have a look at this article to improve the QML performances in your program