I have a family member who hasn't updated her Windows machine via Windows update in years. She barely uses the computer. It is a low end model but not a terrible one; it's a Samsung from 2013 that came with Windows 8. It not only got really slow after at least 3 years of no Windows update, but trying Windows update won't even pull anything from the internet. I reset the entire thing to factory settings, and it persisted.
Download the latest available windows 8.1 (not 8) from here: https://www.microsoft.com/en-us/software-download/windows8ISO
Or better download windows 10 from here: https://www.microsoft.com/en-us/software-download/windows10
Then do a clean install and check how it works, if still slow then replace the HDD with an SSD. And not regularly updating does not cause the computer to slow down.I also have a windows 7 desktop at home which I never update but it is still very fast (Core2Duo).PC's just do become slow over the years because of all the applications a user installs over time and thus doing a clean install will help. Also using a good antivirus is important and hardware issues also can cause the computer to become slow over time.
There's also perception.
Before SSDs people would expect a computer to take several minutes to 'pretend' it was actually ready to go, then it would take another 2 or 3 minutes before all the background tasks & startup items were fully loaded, long after the desktop was visible.
These days, if it doesn't start up in 15 seconds, we want to know why!
My current machine started out life with HD, now has SSD. I've got so used to it that if I try anything 'experimental' for which I need to drop in an HD instead, I rapidly run out of patience waiting for it to do what should be quick, simple tasks.