what makes an emu faster than the other ?

[Squall]

Java does not allow dynrec, therefore it will never be good for emulators.

Another problem are emulators created for "documentation" purposes. Thus, accuracy is more important than speed. Some operations are invalid in real hardware and no game executes these operations. Some faster emulators allow these invalid operations. While it does not break real games, some hacks and homebrews may execute it and consequently they may not work in real hardware and other emulator of the same system.

Byuu (creator of bsnes) had a good explanation about accuracy, but he deleted the articles.