Post hoc, ergo propter hoc.
You may rationalize anything. That does not change the fact that reinstalling the OS is a colossal waste of time. Having owned Macs for twenty years, I know beyond a shadow of a doubt that MacOS X does not take all that.
If you suspect a system slowdown, then run File System Check (fsck -fy) from the commandline in Single User Mode. Do this and keep at least 10% of your hard drive capacity available as free space and your system should purr like a kitten.