One issue I've seen mostly with laptops (although also with desktops in dusty environments), is that the fans get clogged with dust, grit and hairs that cause the machines to overheat and then the CPU goes into thermal slowdown mode. So from a cold start after installing the OS the machine is cool, after a couple of hours of installing updates the machine has reached a toasty temperature and the CPU throttles down. Looks like it's the OS, but it's really the hardware.
Look at the event log in admin tools and see if you are getting CPU throttle notifications.
Hard to clean the fan on most laptops, and may not be worth the time on many old desktops.