As the Palm folks have shown us, it's not a performance problem to emulate a system on one twice as powerful, as long as you rewrite the OS in native code; the problem there is that if you miss a querk, something may not work right. Also, if you aren't the original author of the OS, that may be quite a job. Taking that into consideration, emulation environments written by those responsable for the original system are inclined to fewer problems.