I'm running the latest release of OS X (10.8.2) on a 2.4 Ghz Core 2 Duo MacBook Pro from 2007. The only thing the OS has done is improve in speed over its predecessors. I am not sure that it is entirely accurate to say that the Apple development team got SSDs before we did because there was a noticeable difference in speed on my laptop that was definitely appreciated.
I had the opposite feeling of my old mbp. Every OS upgrade seemed to slow down until I got an SSD and more ram in my new laptop. Which is expected to some level. To say it got faster each iteration is a bit of a stretch.