I would not expect lack of use by itself to make a computer run slower. With the exception of the cooling fans and the hard drive, all the rest is solid state electronics. If the hard drive was running slower, due to any HD fault, I think you would be aware of a HD problem.
It may be psychological ! Not that you are going crazy but, because you normally use a more modern, faster, machine it just seems slower when you go back to it. It may always have been slow in the first place ! We have an elderly Toshiba Satellite running XP as a spare computer, it used to be my wife's before we replaced it. It was always was slow, but when I go back to it now I find myself swearing at it ! It is incredibly reliable, just don't ask it to multi-task, but, hell's teeth, it's slow - but then it always was !