Gregory's ServerHow dare you?
But in fact I can imagine that clogged up air vents (is this the right word in this case?) full of dust increase thermal problems resulting in lower frequencies by some hardware protection.
And degraded batteries also lower the time without power supply.
Yet, my 10 year old PC (same for laptop) is still quick as on its first day when firing up mutt or rendering a web page (or mail) with lynx or links. Even when a notification for a new article takes up 110kB or 2614 lines. (For a simple "XYZ wrote something new. Click here to read it: <500 char long link>")