The hardware in an Apple PC is not significantly different than any other computer.  In my 25 years of working with computers I've fixed them all.  Hardware and software.  Computers slow down because software gets outdated, "dirty", and messy.  Quite often a clean install of an operating system will return a computer to original specs.  That will often still seem slow when compared to a new machine.  Hardware also slows down but that is generally heat related and can be minimized/eliminated by keeping the machine clean.  Blowing the fans and heat sinks out periodically helps.

For the record, I'm on my second MacBook. I purchased the first in 2012 and it lasted 9 years.  I upgraded HDD, memory, and battery and would still be using it today if the charge controller and battery hadn't quit for a second time.