A 2008 computer running 2011 software will be more heavily loaded than a 2008 computer running 2008 software, which will be more heavily loaded than a 2011 comp

A 2008 computer running 2011 software will be more heavily loaded than a 2008 computer running 2008 software, which will be more heavily loaded than a 2011 computer running 2008 software - hardware gets faster, software gets less efficient/does more. As to "OS slowdown": I have, in the past, read posts that say that reinstalling Windows made their computer fast again. Whether that's due to less registry cruft or removal of unnecessary programs, I couldn't say, but it does suggest that the effect is not completely based on hardware getting better or new versions of software using more resources.