This Article may Help You, I Face the same questions a couple of years ago. I decided to stick with Java for my own programming experience, since I'm not that good in C++ and my project was to be honest, very simple. As you know, Java is a very spread / wide around the world, tons of docs and libraries ready for you to use, Qt is faster, but you will need to get your hands dirty to do the job.
If performance is your goal, Go Qt. Or redesign your application to hava Java/Swing GUI and C++ programs server side. Anyways here's the link.
Using C++ instead of Java improves CPU performance, sometimes as much as 10-30%. However using multiple threads also increases the amount of CPU you have available. Given using multiple threads didn't help, I suspect your bottleneck is not in CPU and switching language is unlikely to help.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.