I actually have a pretty massive patch for QEMU sitting in a computer somewhere (with a pretty massive bug) so I know QEMU pretty well. QEMU is actually very fast if used correctly. Android is not using QEMU correctly. I'm not really sure what they're doing wrong but if I can virtualize a VMM which then virtualizes another OS and the interaction is essentially real-time, basic ARM and Java should not be out of the performance target.

P.S. If you think QEMU is slow, don't even think about Bochs.