I already showed in my benchmarks that my block editor is faster than all block editors on the market - even more than those that uses native frameworks. And there are ten of thousand of lines of QML code (and round the same of C++ as well).
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.
If you ever run into trouble with execution of JS slowing down your Qt/QML application, you are using way too much JS. The most common performance issues in decently written applications are rendering of invisible items aka overdraw (especially on very weak embedded SoC GPUs) and slow startup time. There is tooling to find these and ways to fix or improve them.
> 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.
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.