Reinstalling Windows XP SP 2 on my computer growing up was one of the first things that got interested in what was going on at the operating system level. I found Windows XP Inside Out around the house, read it over so many times, and eventually re-installed it on my computer for fun.
I was floored to realize how much faster things were. No wait time for a Windows Explorer window to open. Everything was snappy, even after reinstalling games and antivirus. It blew my mind.
I still think it helps, but with a much longer tail on the frequency required. Windows XP definitely grew slow over time though, whether that was due to installing programs, caches I could’ve cleared with CClearner, or more incremental updating. We leave a lot on the OS over time.