People who are going to use it should read the documentation.
“It depends” and “it’s still slow” are the fairest comments I can make about this.
You can't claim something is slow without showing empiric data. I showed mine when I claimed programming Qt C++ and QML together is fast. If you claim otherwise, you need to support it with data.
> The most common performance issues in decently written applications are rendering of invisible items aka overdraw
That's indeed what I found as well! Especially, these hidden items consume a lot of unnecessary RAM. What tools do you know for Qt/QML that can help with this issue?
For another perspective and more details, RenderDoc (or another frame debugger if you have one) is a nice tool as well.
Also don't use Rectangle { color: "transparent" }, use Item {}. An Item has geometry, but doesn't render anything. A transparent Rectangle probably also doesn't render, but it's still (at least slightly) more resource-intensive and makes you look like you don't know what you're doing.
Use Loader, StackView and visible liberally to disable stuff that isn't currently relevant. If unloading causes trouble with lost state, you may be carrying too much state on the QML side.
Not really, if you avoid writing Javascript code in your QML components, than most of your executable will end up being compiled C++ code. If you do write Javascript code in your QML components, than it *could also* be compiled to C++ code using the QML script compiler[1[2].
> QML apps that heavily implement core logic in javascript would be slow as balls.
The entire point is to separate logic and view where logic is written in C++ and QML simply represents the view (which almost end up being built upon simple primitives that *are* C++ objects). So if you keep this separation you get amazing performance with great simplicity and velocity.
[1] https://doc.qt.io/qt-6/qtqml-qtquick-compiler-tech.html
[2] https://doc.qt.io/qt-6/qtqml-qml-script-compiler.html