I’m using a 2008 2.4 Ghz MacBook (aluminium) and find that it hasn’t run OS X really smooth since Lion, although it became noticeably better with Mountain Lion. Yosemite stutters a lot more in areas that never stuttered before and overall the system is just so much less responsive than it used to be. Performance of games seems to be better too on Snow Leopard, especially games that I’ve been playing since Snow Leopard when I still had an HDD and only 4 GB of RAM (now an SSD and 8 GB of RAM).
I’m running Snow Leopard on an external drive as well and the responsiveness almost brings tears to my eyes. I’ve been tempted by the Windows 10 beta and am impressed with the performance so far, despite the crashes and hang ups. Even on my older hardware, Microsoft manages to get that more performance out of it. It’s just sad that it has come to this. I am looking more and more at Linux now, because I have the feeling that they don’t use the better hardware as an excuse to burden the system more, despite similar end-user features on the desktop.