The only way Windows can slow down with age is if you install and uninstall a lot of software that doesn't properly remove itself. OS X is no different and I've experienced that first hand, especially when running trial versions of software in the OS.
This is just plain wrong. Trash the App and the only thing leftover generally are the preferences. Whatever is leftover certainly won't slow down the computer. This almost seems like one of those "I don't actually use a Mac but I'll pretend I do and say it doesn't work well" comments.
With Windows you just run ccleaner and disk defrag the same way Onyx and at least one application remover and you'll be fine and this slow down will never happen.
Macs don't need to be defragged. Unix essentially does it automatically.