QT is becoming slower ?

I am not sure how QML works, but it suggested that it is interpreted by an engine of some sort like JVM

So , since more and more features have been migrate to QML in QT5, will QT lose its advantage in performance?

And what is its advantage compared to other technologies ? For instance, Java, Html5,etc.

I don't know the current state of QML but languages such as javascript (which is what QML really is) can be compiled into machine code using JIT (Just-In-Time compilation) and executed with full speed. That's basically how JVM works as well. I know that JIT technology is present in Qt so a good guess is that it is being used for QML as well.

You can simply keep using normal QWidgets, also in Qt 5.