Sure, you will take a runtime performance hit if you do a QML UI because all the QML needs to be parsed.
But then again, no it's not all javascript and no it will not suck up all of your CPU. When you write simple bindings (for example someProperty: otherPropert ? "yes.png" : "no.png") the QML engine actually detects these simple cases and doesn't invoke the javascript interpreter when it needs to evaluate the expression, but handles it internally instead. Also if you don't want to do things in javascript you really don't have to as calling QObject slots is super easy. If you stick to simple bindings and write your logic in C++, the javascript interpreter won't have much to do.
Sure, you will take a runtime performance hit if you do a QML UI because all the QML needs to be parsed.
But then again, no it's not all javascript and no it will not suck up all of your CPU. When you write simple bindings (for example someProperty: otherPropert ? "yes.png" : "no.png") the QML engine actually detects these simple cases and doesn't invoke the javascript interpreter when it needs to evaluate the expression, but handles it internally instead. Also if you don't want to do things in javascript you really don't have to as calling QObject slots is super easy. If you stick to simple bindings and write your logic in C++, the javascript interpreter won't have much to do.