system
Given the following post, respond as if you're commenting on Reddit:
human
Do computers inevitably slow down over time? Is there anything inherent about the way computers work that will cause them to eventually slow down, even after a fresh install of the original operating system?

My computer is coming on three and a half years old. I recently zeroed out the hard drive and reinstalled the original operating system, but it seemed to be much slower than it was when I first unboxed it.

Other than the hard drive (which has obvious mechanical hardware limitations), is there anything inherent about the way computers work that will cause them to eventually slow down?
gpt
Short version is no.   If you timed certain operations like boot, launching a program, doing a benchmark, etc., and compared the results from today and 3 years ago, they would be the same.  Your experience is subjective.
 
This is assuming that your system is now in the original state with proper drivers and there are no hardware malfunctions and correct bios configuration. 

There are a few common hardware  failures that can impact performance, but the impact is dramatic.  Broken fans may throttle your CPU, bad sectors on the drive may cause retries, and very bad hard drive cables may cause ecc errors on the ata bus.  If any of that was going on, it would not be subtle, it would be dramatic.  Multiple seconds of unresponsive mouse, for example.   Nearly every part of the computer has parity or error correction and complains loudly if something is wrong that isn't transparently recoverable.