The first ARM desktop computer, the Acorn Archimedes, got quite early on a PC emulator which, if I recall correctly, emulated a 80186. The ARM 2 processor, running at 8 MHz could emulate this processor at close to 5-6 MHz (again, if I recall correctly).
From: http://chrisacorns.computinghi...
"In use the Archimedes PC Emulator program gives quite acceptable performance if you don't want to go too fast. While the hard disk access is extremely fast, the computing speed is only average and the screen display speed is slow."
And it gives the 'computing index' performance as about 1/10 of an AT PC. That's pretty much my experience of PC emulators; for apps that spend most of the time waiting for user input, it's fine, but anything that requires real computing power needs a CPU that's about 10x the performance of the CPU you want to emulate.