I'm a bit worried about performance with QML since already the UI is a bit slow (not much, but noticeable) and switching to JavaScript might negatively impact that. We'll see.
But, as I has an 10 y/o laptop with integrated, any heavy JavaScript GUI apps (based on QML/Electron) just slowed down performance instead of increasing, in comparison to Qt-based or native GUI apps.
Yes, I know as an owner of Symbian 9.x device ;)
But, here I literally means that actual QML coding style stuck to requirement of modern GPU with OpenGL 2.x (GL ES 2.x) even OpenGL 3/4 support.
My laptop has integrated GPU mostly limited to OpenGL 1.x, so all those CSS/QSS animations, which depends on higher OpenGL features, just blows my CPU/GPU into flame.
1ghz embedded devices with semi-good GPUs*
e.g. the GPU in a Pi 3 blows some older intel GMA chips out of the water, and those are still quite present "out there". Even a 4ghz pentium 4 with such a GPU will feel slower to the user than a 800mhz Pi (which in itself, is not what I'd call enjoyable to use).
If the app is badly optimised it's going to be slow regardless of the rendering method.