Pretty sure Snow Leopard was a release aimed at doing just that, rewriting a lot of the core system libraries for x86_64 and also clearing house on a lot of old cruft. No new real features were delivered with the release.
Then Lion came along and slowed things down to a crawl. Definitely the slowest OSX release I've had on my old macbook (circa '08). I eventually downgraded back to snow leopard until mountain lion came out, which was noticeably faster than Lion, but still not quite as smooth as snow leopard. But now Mavericks is back to being pretty good.
All-in-all, still not a very straightforward 'improvement' path performance-wise -- at least not from the end consumer's point of view.
