Linux has worked pretty hard to optimize fork(). This doesn’t mean that fork() is a good idea.

Windows does not historically depend on fork(), so there was no native fork(), so Cygwin kludged it up.

Actually, there is a native fork. There had to be, as POSIX personality support was a part of the Windows NT 3.1 design. What there wasn't was a Win32 form of fork. The Native API for Windows NT allowed it quite straightforwardly.