It's important to note the difference between emulation and virtualization. Most methods of running a different (or the same) OS within a host OS uses virtualization, which means it has access to all of the computer's native hardware instructions and will run almost as well as if it were running natively. Since most Windows/Linux PCs and all Macs run on Intel hardware, they're able to run in a virtualized environment natively with little to no slowdown.You can however run Windows emulators on Mac or Linux computers to play it on those systems. Your mileage may vary doing that though, as you have to support the resource usage of both the emulator and the client.