(considering where the bottlenecks probably are I doubt that the language itself is the problem - it's wrapping a browser that is. Even Python/Qt would probably be performant enough in this context)
I keep meaning to dive into it to learn it now that my C++ is better, but what stops me there is that I don’t quite get where QtQuick + JavaScript fits in, or what parts of Qt I should be using / putting time into learning. For other languages, I don’t want to use an ‘unofficial’ binding in case it breaks.
QtQuick + JavaScript puts JavaScript into its original place that it had at the times of the old web: oneliners to bind stuff together.
Also there is probably a confusion from Qt having two distinct GUI systems: QtWidgets and QtQuick.
It's important to take into account the dates when the learning materials were published. New features change the way how the same thing is done (while keeping the old way still compiling and working). Specifically about the STL there are old books from Meyers. To not get lost in time there is the "C++ Standards Support in GCC" site of the GCC compiler which enumerates all the new features that were added during the post-2011 accelerated development period. The modern C++ has seen no books so it's may be useful to just watch some talks by people who develop the language, standard library, compilers or who have experience in using it: Sutter, Lavavej, Stroustrup, Alexandrescu, Carruth, Hinnant, Vandevoorde, Niebler for example.