What I am saying is that where I work, we support commercial systems on a VERY large scale - tens of thousands of systems at various ages and in various models. In proportion to this, we see all kinds of problems every day and part of my job is to figure out as many of them as possible. I also spend a lot of time in public computing facilities so I know a little about that. My experience is probably broader than someone who is just overclocking his own PC.
I have never seen a slowness problem on a PC that can be attributed to the HDD "going slow" and you have no direct evidence to support your opinion that HDD's can "go slow" after any amount of time - it sounds like hocus pocus to me and there is likely an underlieing issue which isn't understood wherever you are basing your perspective from.
Insufficient cooling can shorten the MTBF of a drive, however a drive doesn't just slow down... It either works, or begins to show signs of failure, one of which is not a general system slowness.
