In general, components with no moving parts will end their life abruptly, and will just work one day and die the next. They don't "get slower" as would a battery for instance.
That said, some components do wear down with time, most notably mechanical hard drives, their magnetic properties may wear off after a (very long) while, which causes the hard drive to do more work to succeed at reading/writing data, thus making the system slower (but this usually isn't seen as a well-maintained hard drive far outlives its practical life)
Power supply units can also degrade with time, and start having fluctuations in their power output which can cause certain electrical components inside your computer to behave erratically and possibly perform worse (or just outright burst into flames, if you are unlucky).
Another likely possible cause is overheating, many non-tech-savvy people use overheated computers without even realizing it, which significantly slows down the system as a CPU equipped with thermal throttling will, well, throttle itself to protect itself from damage. It would be sweet if OS'es provided some sort of clear software alert to inform the user when this happens, but I guess this requires standardization of motherboard thermal sensors, good luck with that.
But I would agree that in most cases, this feeling of slowness is artificial. In fact, when we use the computer, we don't generally think about how slow it is, because we have nothing faster to compare it to (if anything, we think about how much slower computers were in the past). However, the instant you touch a computer with better, faster, newer hardware and software than the one you use, you have a point of reference, and you start expecting your own computer to perform better, hence the feeling that it is running slowly.