You would think that an st emulator would be quite quick on the amiga, more or less the same machine. Only have to emualte the sub-standard graphics chip
It's easy to virtualize the operating system. It's a lot trickier to emulate the machine, at least if you want to be compatible with ... well most stuff. That's a huge difference. The interrupt controller, the blitter etc. Even the graphics hardware becomes slightly tricky, because the plane interleave is difficult to achieve on the Amiga w.o. having to shuffle the bitplanes manually (copper interleave afaik does longword interleave, while the ST uses 16 bit interleave). Even though it has the same CPU, you'll probably have to emulate it if you want some degree of hardware compatibility.