I agree with you, but i'm pretty sure the main reason its slow is not java but these two things:
-Serial communication loop which runs several times a second
-Drawing the sensor chart and data
Sure, you wont have that much overhead if you're not using Java or .NET but QT4 is primarily designed for classic gui application which usually don't include many graphical elements with a high refresh rate.
For this kind of stuff it would be best bet to use opengl which can be
integratedin a qt4 application. But you would still have to separate the display stuff from the serial communication data or at least make it non-blocking.
A modular approach is also important of you want to go multiplattform because it might be necessary to use a completely different libraries for lowlevel stuff like serial communication.
Regarding python/c++:
QT is a gui framework, it has bindings for many languages. It seems most people use c++ but there is also a considerable amount of projects using pyqt. Programming languages are a matter of taste, but you will have a hard time finding people that think writing code in c++ is more fun than python.
-> The overhead of learning qt would be much higher than switching from c++ to java or python, because if you use an extensive framework much of the code will be written with the specifics of the framework in mind.
Look at the code for a qt application with c++ or python, you'll see what i mean:
https://github.com/void-main/CuteEditor ... window.cpphttps://github.com/firefoxmmx2/pyqt_tes ... Example.py