Performance was the reason I wanted to try out C#/.NET 4.6.2 and newer which also is much easier to work with than Qt. Performance is different in the later as it starts using all cores on a CPU. What trouble me with both solutions is however the effort it takes to make dark themes. I ended up using a minimum of time to write a drawing engine and a lot of time fiddling with user controls.
QML is not better as you basically need to do the same job, but as I learn QML I also see that it get easier. On QML I might actually struggle with what was easy in Qt/C# – the drawing engine, but let’s see. One step at the time.