Sometimes you'll see a little cylindrical dohickey on the power or monitor cables. If I recall correctly, it's called a balun. If you cut it off a system that is acting sluggish, you'll often find that it has built up so much deposits from the garbage in the lines that it is practically a solid hunk of metal. The balun is your computer's equivalent of a fuel filter. You may need a new one.
/jk
Not to mention the absolutely vital step (after replacement of the balun) of refabulating the widget, making certain that the power lines are set to electron flow rather than hole flow.
/even more obscure USN jk