I'm not a Java programmer, but as far as I know the JIT parts of the JVMs are very fast these days, almost as fast as compiled langauges if used properly. Java programs that are slow are mostly slow because ... 1) Java programmers often program in a way that's prone to be slow, they generalise things a lot, they write programs that produce a lot of objects and need a lot of GC. 2) The Java GUI libraries Swing & AWT are quite slow, especially if used in the way they're normally used.
Regarding wxWidgets I don't think adding little bits of C++ to C programs is a very good idea. The main program is still C and faces the disadvantages of that language. A small portion of the program is C++ and this requires a C++ compiler bringing with it the problems of compatibility between compilers and versions of compilers. Also the C++ bits could be difficult for C programmers to understand. I.e. the disadvantages add more than the advantages. Still, I don't write any of this stuff ... do whatever works.
