>That's bullshit, at least on AMD64. "Because the full 32-bit instruction set remains implemented in hardware without any intervening emulation, existing 32-bit x86 executables run with no compatibility or performance penalties" [0]

Which is true if you're running a 32-bit kernel on top of the 64-bit processor.

If you're running a native 64-bit kernel, then the kernel needs a 32-bit emulated layer that runs between the native kernel and the userland application that handles virtual memory address translations(32-bit executables are still limited to 4 gigs of RAM, but can be physically located anywhere), system call translations, etc. That's a potential performance hit.