It's a lot more than that. When Cygwin is built, the translation between to POSIX is handled via Cygwin.dll with the various tools recompiled against that file

It's a lot more than that. When Cygwin is built, the translation between to POSIX is handled via Cygwin.dll with the various tools recompiled against that file. This version is taking the native Ubuntu binaries and executing them directly against the Windows API via a real time translation later. The difference is like playing a game using virtualization technology vs. WINE. As the complexity of the game increases, the former begins to slow down and break.