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.
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.
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.