>a64bit kernel with 32 bit userspace is reasonably common.
. It is common on arches where 64-bit is considered expensive - such as sparc64. But on x86, people have obviously made sure 64-bit mode runs at least at the same effective speed because otherwise it would be harder in selling it. Plus, in precompiled distros, the 32-bit objects often do not contain SSE/SSE2 because it is not guaranteed to be available in all cases (and in fact, libvorbis/oggenc will speed up by 17% when adding -msse -mfpmath=sse on ye olde 2003ish AMD Athlon XP); on 64-bit however, SSE is always available, so naturally it comes faster compared to precompiled 32 bit objects. (IOW, Gentoo is exempt because you can add -mfpmath=sse at your leisure.)