Java really isn't as slow as many would make you believe.

http://kano.net/javabench/
http://www.idiom.com/~zilla/Computer...benchmark.html

Certain operations can be slightly faster, whilst others are upto 10% slower. So for many consoles e.g. Snes, Genesis, GBA you should be able to get away with Java.

The G1 managed to emulate the three consoles above rather well.