A pure C/C++ emulator is not likely to be that much faster than a Java one.
In my experience, emulators are not really java's cup of tea. While it's true that most emulaters written in C/C++ have some inline assembly, it's also true that because you have much more low level control in just C/C++, C/C++ based emulators are generally quite a lot faster than the java ones. This doesn't necessarily make C/C++ quite a lot faster than java *per definition*, but a better choice for creating emulators if performance is a major concern.
Look at the emulator 'Modeler' for example (emulates Sega System32 arcade machines). It's completely written in C++, no fancy dynamic recompiling CPU cores (just interpreters), and it emulates possibly the most complex 2D video hardware ever. It runs quite acceptably on a 450Mhz P2.
You probably won't come close to that kind of performance if you did it in java.